Lets not forget that Windows 7 is also involved. We do lots of testing at my work, and i have noticed at times that Win7 sitting at idle consumes 1.5 meg just to keep happy.
I don't suppose that the problem system is duel core? If so, you could move your security packages and some other system processes to a dedicated processor.
If it were me, I'd bump the memory up to at least 4 Gig, XP can handle most of that, would allocate some more to other system proc's.
My 2 cents worth..
Seekerii
aka Stormlord