Are you getting any error messages that appear when installation fails? If so, what do the mssg.s state? (A sample.)

If no help avails itself via MS, and I'd probably do this anyway, I'd roll back the machine to a checkpoint when everything was working well. (post when SP2 was installed).

Was SP2 installed before or after your AV, Aspy, Firewall, etc.?

Aspy would probably be the one I'd select to remove, first and temporarily, for diagnostic purposes.

The thing I'd be looking for or trying to determine is whether one of the installed programs (and/or its underlying files) is causing some type of conflict interfering with the ability to do further installations.

For some reason, anti-spy. apps. have been tending to mess up people's machines.

That's the short "what I'd do" stuff.

The more tiresome one is to look at the computer's logs of, i.e., Events.

I'm booting my laptop with XP; I don't remember whether XP has the ability to go to the machine's Performance Information, then, to view Probs. & Solutions (as can be done with VISTA).