i've installed cs4 with Avira AV turned on , so i guess there is no general AV issue.
Av like Kapersky, Panda, Norton, Bitdefender etc are far too heavy anyway and they will dig themselves deep into the system - that's why i don't like them. Biggest problem is the human part of the chain anyway.
Several progs of the cs4 respond slowly in some s¡tuations - like tracing bitmaps with flash , but this behavior I've seen on different Systems, even very powerful ones and with W7, Vista, xp, don't know about Mac.
So my guess is that it's like this, no matter what Windows Version you have