To get an idea of how Windows got to be such a mess, think of it as a house that was built on an island in the middle of a lake, deep in the countryside.

Because you're so isolated, you don't need to worry about keeping strangers out -- your security rests on being physically separate from the rest of the world.

So it doesn't matter that the windows can only be latched shut with great difficulty, that locks were picked to match the decor (no ugly deadbolts here!) and there's an extra key hidden under the doormat.

Now take that house and move it into the city. Shopping or socializing no longer requires a long drive; all the distractions you might want are right outside. But there are a few burglars in town, and they all know how easy your house is to break into.

In this case, security means making sure that nobody can get in the house unless you open the door yourself. You need to hire a good locksmith.

With a new update called Service Pack 2 for Windows XP, Microsoft is trying to perform the same repairs, making software once built for isolated desks safe on the crowded, bustling Internet.

Service Pack 2, "SP2" for short, is Microsoft's most important release since XP itself. It aims to stop viruses, worms, browser hijackings and worse by including security features that people had to add and adjust on their own. (Users of Windows 2000, Millennium Edition, 98 and 95 will still need to do that, since Microsoft has no plans for a comparable update of those systems.)

The most important part of SP2 is an new firewall program to stop break-ins by network worms such as Blaster. Unlike XP's earlier firewall, this one is turned on automatically and protects every connection on a computer -- even if you already have another firewall active. It also watches what your programs do; if one wants to open its own channel of communication with the Internet, you'll need to approve this action.

The need to make this choice for potentially dozens of programs, even Microsoft's own, can be a drag, but the decision should be fairly simple: If you recognize and trust the program, it should be safe to "unblock" its access. But if you've never heard of it, keep blocking it unless things stop working.

Automatic system updates are just as important in Service Pack 2. The first time you boot up a computer after installing SP2, a can't-miss, full-screen alert asks you to allow Windows to download and install Microsoft's security updates automatically.

A new Security Center control panel provides quick access to firewall and automatic-update settings, and it checks for active, up-to-date anti-virus software. Though anti-virus protection is essential to Windows security, SP2 doesn't include any; you must install your own.

As part of SP2, the Internet Explorer Web browser now -- finally! -- blocks unsolicited pop-ups. This overdue step eliminates many intrusive ads (yes, The Post's Web site runs its share) but also stops hostile Web sites from tricking users into downloading unwanted programs by barraging them with pop-ups.

To police browser-hijacking attempts, IE now restricts the ability of Web sites to push "ActiveX" programs on visitors; the default choice is to decline an ActiveX program, and you can reject all future installations from a Web site with two clicks. ActiveX should be retired -- this inherently insecure Microsoft technology grants a Web site unrestricted access to your computer -- but these changes should at least make a hijacker's job tougher.

When you use Internet Explorer to download new software, Windows will ask if you're sure you want to run each of these programs, even if that's days after its download.

Because of these and other security fixes, I did sometimes have to reload a "download now" page to convince IE that I really wanted the file -- a small price for a safer browser, albeit one still behind such competitors as Mozilla Firefox in its capabilities.

The Outlook Express e-mail program, meanwhile, no longer allows any access to programs sent as file attachments. Hiding a program inside a compressed "zip" file won't work either -- you can decompress the archive, but Windows won't let you run its contents.

This may frustrate users who send greeting cards and other little programs in e-mail. I don't care. The cost of mail-borne viruses is too high, and Microsoft did the right thing in placing security over convenience. (Imagine if it had made this trade-off four years ago.)

Many non-program attachments, such as Microsoft Word documents and MP3 audio files, also require a second click to confirm that you really want to open them. (Pictures didn't require that extra step.)

Outlook Express also stops the display of Web images in messages, a common trick spammers use to see who opens their junk.

Service Pack 2 does include one bonus feature that isn't strictly security-related; it now lists the signal strengths of each available WiFi wireless connection, an obvious feature that Microsoft inexplicably left out before.

Beyond these visible changes, Service Pack 2 folds in numerous alterations to the inner plumbing of Windows. Such unneeded features as the Messenger Service, which spammers exploited to broadcast official-looking pop-up ads, are now shut off, and others are exposed only to a local network. With SP2's firewall shut off, however, I did find that it left two network ports open for no apparent reason.

People running computers equipped with 64-bit processors get extra protection against "buffer overflow" errors, a common tactic used to sneak hostile programs onto a computer. Service Pack 2 can tell these chips to enforce "no execute" rules that prevent a program from running in a block of memory that isn't specifically reserved for use by programs.

Despite all the surgery Service Pack 2 conducts in the guts of Windows, all four of my installations worked. A Dell desktop needed 30 minutes; two others took closer to an hour, and an older IBM desktop needed two hours, counting the time needed to start from scratch after a first install was halted by a mysterious reboot. The only program I found that did not function afterward was a specialized networking utility.

Many of SP2's Internet features, such as its control over downloads and attachments, don't work in other Web and mail programs, but developers of those can add support for them.

Service Pack 2 still can't save gullible users from themselves, though. And since it continues to grant people "administrator" access to a computer, any one mistake can take down the entire machine.

This leaves Windows XP at a continued disadvantage compared with such competitors as Linux or Mac OS X. (Programmers call the idea of giving a user no more power than needed for the job "the principle of least privilege"; the same logic comes into play every time a parent gives a kid a $20 bill, not $50, before sending him out to pick up a pizza for dinner.)

Service Pack 2 is a free update, but it's not easy to get -- yet. A 266-megabyte download is available at Microsoft's site (go.microsoft.com/?LinkID=806688), while users with automatic updates enabled will have a smaller version sent to their PCs over the coming weeks. Around the end of the month, SP2 will be available on CD-ROM; to Microsoft's credit, it will ship these CDs at no charge.

Computer manufacturers should be able to add this update to their systems within a month or so, Microsoft says. I would like to suggest that any firm that isn't pre-installing SP2 by November has no business selling home computers at all.

Individual Windows users bear the same responsibility: If you run XP, you need to install SP2. Period. Loading a system update this big is never risk-free, but the far bigger risk is to keep stumbling along with an unpatched copy of Windows XP. Ask a computer-savvy friend to install it if you must. But don't wait for the viruses and worms to stop coming. They won't.

Living with technology, or trying to? E-mail Rob Pegoraro at rob@twp.com.

Service Pack 2 contains many changes: The Internet Explorer Web browser now squelches pop-ups and makes it easier for users to stop Web sites from installing new software on their PCs; the Outlook Express e-mail program blocks access to programs sent as attachments; and a new interface simplifies setting up WiFi networks.Firewall programs protect against online break-in attempts by network worms. Service Pack 2's program for Windows XP is switched on automatically and is much easier to configure than an earlier version.Service Pack 2 advertises Microsoft's software updates prominently and encourages users to let Windows install these security fixes on its own as well as downloading them automatically.Service Pack 2 doesn't include an anti-virus component, but its Security Center warns users if an anti-viral utility already installed isn't switched on or is out of date.