It's a custom-built PC. If it helps at all, it used to work ok without the problems I've mentioned.
I have no virus protection. Windows firewall is enabled - would that affect my performance?
I don't know how much detail you're looking for, but I've got Adobe Production Suite, and several other small utilities like Stuffit, Itunes, VST's, divx codecs, Serious Magic ULTRA, firefox. Even when none of these are running as an application, I still get problems.