i've no Idea But its happen on a number of occassions, i've spent many nights repairs and reinstalling windows xp on friends machines.
Maybe people were installing stuff that changed there DLL's are modified them and thats why they were remove.
Now they just use MSAS/WD. Seem to enjoy it, as they can set it to inorge items/allow items.
I use to be big fan of Adaware, Use to use it on every system we have in work. Which i had the fun of installing. Soon it was replaced by MSAS/WD.
TBH Spyware
i never get any now that i been aware of what to look for and stuff, i don't even get viruses anymore(unless i getting no CD crack etc. but AVG picks it up striaght away.)