Adding programs to a Windows PC is an act that violates a natural law of modern computing: If it's not broken, don't fix it.

Software installations usually work fine, but they remain a mysterious, risky business. An installation may fail visibly or invisibly, or cause nasty problems that surface long afterward. You might not get much choice of where and how a program embeds itself in your PC, and you won't have any choice when you are asked to accept dense paragraphs of legalese.

Worse yet, installing software makes you say "yes" when a tech-support rep asks if you've changed anything on your PC.

Why is this so hard?

PC architecture is endlessly variable, so no two PCs -- or even two configurations of Windows -- are alike. And that's before each PC's owner has installed different programs, each of which can introduce itself to the Windows registry (the database of system settings) differently.

While very simple programs can be installed by dragging a file into the Program Files directory, most applications consist of multiple files, some hidden. Installer programs make sure each of the files winds up in the right place.

Little in Windows can stop a rogue program from stepping on critical files. That helps explain why installers tell you to close all other open applications.

That nagging message is not just the product of computing superstition. In Windows, multiple applications share files, called dynamic link libraries, to handle common tasks. Installers often update those files, and changing a DLL being used by another program can cause errors or crashes.

Ensuring that only the installer can touch a DLL reduces the odds of a failed installation. Closing applications also frees memory, allowing the installer to work a little faster.

You can use the time you save to digest the legal gobbledygook in the new program's product license. Those legal agreements, usually in tiny type inside tiny windows, are an issue unto themselves. While it's tempting to click "I accept" and get on with things, it's worth reading the gory details. For example, some free-to-download programs will put other software in the background to download and display ads on your desktop.

The required post-installation reboot, like the requirement to close other applications, allows Windows to replace files that were in use during the installation and make necessary changes to system settings. (That is why a post-installation reboot may take longer than usual.)

Will all this get any better?

It might. Installers usually have run off scripts that robotically plant specified files in specified locations. Microsoft's Microsoft Installer "engine" (also called Windows Installer) is more flexible and reliable.

Microsoft Installer can recover from a failed operation, repair damaged files, and install and remove components of an application. It also allows parts of a program to be installed on demand, without the need to exit the program first. (You can see this option when you install a new version of Microsoft Works or Office.)

The code for Microsoft Installer is built into Windows 2000, Millennium Edition and XP. But on older operating systems, adding a program that uses Microsoft Installer will require you to -- you guessed it -- install the installer.

The two leading installer-software developers, InstallShield and Wise Software, now strongly encourage their customers to use their Microsoft Installer-based products instead of older, script-based tools.

But installer vendors, application developers and even Microsoft are fighting an unpleasant truth: PC architecture is undisciplined, so any improvements are likely to remain marginal instead of fundamental. You still need to be careful about what you plant in your PC garden.