Old spyware causes the new Service Pack 2 to freeze up some PCs

Spyware already on computers is causing problems for some who install the new Service Pack 2. SP2 is largely aimed at making Windows XP more secure, but it appears that if you already have a bunch of spyware on your computer (and you certainly do if you don't use a spyware killer) then you may have some trouble.

Barry Goff, a group product manager at Microsoft, said some spyware could cause computers to freeze up upon installation of the update...
Programs such as Ad-Aware and SpySweeper can scour computers for spyware. Microsoft recommends that users clean their PCs of spyware and back up their data before turning on the auto update feature that automatically downloads Service Pack 2, or SP2.

WaPo article here.