Windows XP is really old, and we would suggest that you don't use it unless you really have no option. For the most part, however, that age doesn't really manifest itself. Sure, the operating system is missing the security features, hardware acceleration, and built-in support for things like USB 3 that newer versions of Windows have, but old software doesn't have the same issues as, say, old cars. Old software generally runs as well today as it did when it was brand new.

But Windows XP users have noticed that this isn't entirely true. A bunch of them have found that the old operating system is working considerably worse than when it was released in 2001. The problem is that—especially among those who are still using Internet Explorer 6 or 7—each time you boot your Windows XP machine, it slows to a crawl. There's a built-in process, svchost.exe, chewing up the entire processor, sometimes for an hour or more at a time. Wait long enough after booting and the machine will eventually return to normalcy. But an hour can be a long time to wait.

Loss of horsepower and trouble starting up are common enough problems in old cars, but we don't really expect the same things to happen on old PCs.

It looks as if Microsoft has figured out what the problem is—though not at the first time of asking. It's all down to Windows Update. Machines using Windows Update retrieve patch information from Microsoft's servers. That patch information contains information about each patch: what software it applies to (for example, systems that have been upgraded to Internet Explorer 7 or 8 don't need Internet Explorer 6 patches), what knowledge base article it relates to, and, critically, what historic patch or patches the current patch supersedes.

Windows patches are mostly cumulative. On a fresh install of Windows XP, you don't need to install all of the dozens of Internet Explorer 6 patches sequentially; you can generally just install the latest patch, and it will include all the historic fixes because it supersedes the historic patches that introduced those fixes.

Unfortunately, the Windows Update client components used an algorithm with exponential scaling when processing these lists. Each additional superseded patch would double the time taken to process the list. With the operating system now very old, those lists have grown long, sometimes to 40 or more items. On a new machine, that processing appeared to be almost instantaneous. It is now very slow.

Microsoft thought that it had this problem fixed in November's Patch Tuesday update after it culled the supersedence lists. That update didn't appear to fix the problem. The company thought that its December update would also provide a solution, with even more aggressive culling. That didn't seem to help either. Although the company said that it did test these updates, for one reason or another, its test scenarios didn't reflect the experience of real Windows XP machines.

There is hope that a true fix will be developed. The company says that it is working on a new supersedence logic that will "comprehensively solve this problem" and that this work is a "top priority," with "all the right (and smartest) people" working on it. Unfortunately, there's no ETA just yet.

If the fix doesn't come soon, one suspects that it may never come. Microsoft will cease to update Windows XP in April 2014. At this point, Windows XP users will be able to solve the problem for themselves anyway: they can simply turn off Windows Update forever, since it will no longer update their systems anyway.