Was working on a massively-infected computer the other day, and after cleaning off the evil (Ad-Aware alone found over 1800 things), I was upgrading to XP SP2 (service pack 2) when I got the finest error message I've ever recieved from Windows: "The requested section was not present in the activation context". In a way it was good, because it sounded like it *could* be actual information, as opposed to the usual Windows error: "You screwed up". But this was SP2, a completely scripted install that I had no part in apart from the double-click to start it. Anyway, I was getting lots of other errors as well, all having to do with security ("access denied") and the registry and file system, and I was in the administrator's account, so I ended up just freshening security in the registry and file system the old-fashioned way, when I could have probably saved a lot of hassle by finding the above link before I reset things by hand.
So the moral of the story is: keep your system clean, and it will save me and you lots of time and trouble (not to mention prevent the invasion of your privacy, the stealing of your personal information, and the driving of you to insanity with pop-ups and slow internet).
How to stay clean? Get a good anti-virus product and keep it updated; get a firewall, such as ZoneAlarm; and use Ad-Aware and Spybot weekly (and the first thing you should do when you open them is update them). Thanks.