so far 3/4 vista av programs i've tried increase lag in games hugely. nod32 "can" do and is pretty awful for vista. avast for sure destroyed my ping in CS:S, up to 45, turned off real time monitoring(firewall and internet stuff was already off, but file realtime monitoring) and boom, back to 6-7ping from nice london servers. errm, something else was bad too, mighta been trend(bluescreens a lot in vista anyway).
at the mo i've got avg installed but not run cs:s for a bit(one of the few low ping needing games i play lately) so not sure if its affecting lag or not. might have a game later to check. turn off firewall(if 3rd party, windows own seems to not create an issue for me), try turning off real time monitoring, errm, if not that then, well i was without the AV issues running cs:s with exact same ping as with XP and i think lots of other people are fine with CS:S in vista.