Windows Rot?

Can anyone explain why Windows seems to "rot"? As I write this, my main PC is reformatting and reinstalling Windows XP due to the fact that common operations seemed to slow to a horrible crawl. This process is only made worse by the numerous device drivers and applications that need to be reinstalled, and in some cases, re-activated. Why do I have to spend an afternoon a few times a year to keep it running?

I haven't noticed or read about any similar phenomenon on other OS's, so what is so special about Windows that causes such problems?