It works a lot like gravity--slow and almost imperceptible at first, over time it will forcibly return even the fastest speeding bullet back to earth. But there's nothing natural about how a PC, no matter how fast it runs out of the box, can eventually feel no quicker than an antique machine long since exiled to the attic.

People who purchased a new PC last year have probably already started to notice this performance degradation, though they might not have any idea what's going on. It's not the colder weather, the lack of daylight or the dust that collects inside the computer: It's a flaw in the way Windows works.

The problem relates to how programs move in and out of Windows. First of all, a program isn't one monolithic object; it includes numerous little chunks of programming code, called DLL files (short for "dynamic link library"), which are socked away inside Windows' system directories. Second, when you install a program on a computer, it also loads a set of instructions inside Windows' registry file. Think of the registry file as a combination road map and traffic cop for system resources; these instructions tell the registry, and therefore Windows as a whole, where, when and how to run a program's DLLs.

And when too many of these instructions clutter up the registry, the system begins to slow. Imagine you are the system's central processing unit and you need to drive between two cities. All you need to know are the main avenues or highways, but the map you have is littered with information about side streets, blind alleys and routes to other locations, all of which takes time to process and sift through. The problem increases as more programs are added.

Matters worsen when you uninstall programs from your system. Many programs fail to clean up their DLLs after them; many more also share DLL files with Windows or with other programs. You can see this happen when you uninstall something via "add/remove programs" in Windows' control panel; an alert will flash that a file may be used by other programs, and would you like to keep it around just in case? Clicking the "yes" button and keeping the files around is actually advisable here: When in doubt, don't delete.

But in the wake of all those programs is a morass of orphaned DLL files, many of which give the computer useless instructions or allocate memory for programs that no longer exist. This slowly eats at system resources--the amount of processing power and memory available to run programs--until your software bogs down or refuses to run at all.

Shawn Sanford, group product manager for Microsoft's consumer Windows division, acknowledged the problem in an e-mail message but noted that other developers' practices can make things worse. "Microsoft has made strides in Windows 98 and in future versions of Consumer Windows to help with this issue, but this is truly an industry issue," he said. "We are also working with the industry to look at how you solve this issue, whether it be a standard install or un-install method that prevents this."

How bad can things get? On one relatively new PC--a six-month-old Pentium II with a history of frequent installs and uninstalls--I couldn't even get a game's CD-ROM to be recognized. The computer kept on reporting "read errors" on this factory-fresh disc. Investigating further, I discovered that phantom registry entries and old programs' leftovers had choked this machine--only half of its system resources were available, so that when Windows tried to access the CD-ROM drive it ran out of memory, which was then displayed to me as "read error." (To check your own computer's system resources: Select control panel off the start menu, click on "system" and then the "performance" tab.)

Short of not installing any programs, unfortunately, there isn't much an individual user can do to prevent this problem. Editing the registry "by hand"--altering it with Windows' built-in registry editor--is courting disaster. This file can span thousands of lines, and changing one parameter in a critical area can hobble your entire system.

Reinstalling the entire operating system is an equally ill-advised, "we had to destroy the town in order to save it" remedy. The new system will be faster, but you should factor in the staggering amount of aggravation and lost time involved in reinstalling every single application. Plus, you'll soon start to see the same old slowdown once all of your word processors, spreadsheets, Internet browsers and games have been duly implanted.

The best solution is a third-party program that automatically checks and edits--make that, decontaminates--the system registry. Two popular options are Symantec's Norton Utilities 2000 (Win 95-98, $50; http://www.symantec.com) and Mijenix's Fix-It Utilities (Win 95-98, $50; http://www.mijenix.com). Of the two, I prefer Fix-It for its ease of use.

These registry editors work by scanning the registry and looking for DLL files that no longer do anything useful. Then they allow users to delete this detritus. The Fix-It program is particularly good, giving users a color-coded chart. Windows' own system files can't even be selected, so you won't accidentally nuke something you need. The Norton program's interface for this is a bit harder to learn, especially for neophytes.

With either of these utilities, you have to remain vigilant. This is like cleaning the bathroom floor--an unpleasant task that has to be done every month or so to avoid worse unpleasantness later on.

My experience with another older PC proves the point: Repeated software installations and removals had robbed the computer of almost a quarter of its resources, causing daily crashes. After swabbing away 2,398 unused registry entries, system resources jumped to 96 percent free--liberating the computer from its crash-and-burn cycle and even speeding up simple operations such as booting up and shutting down the computer.

But one month later, I had to delete 231 more DLLs to keep things up to speed. Like gravity, this situation never ends.