First, you can't be 100% sure that you're clean - the executable could have downloaded and activated a rootkit, which pretty much renders process explorer & monitor useless.
Second, if you don't want to run Vista, at least consider running under a non-privileged user account. It's more bothersome on XP than on Vista, though. The alternative would be using something like "dropmyrights" on all internet-facing applications (browser, mail client, ...) but that won't stop you from malware if you accidentally(?
) double-click random executables. Dunno about Vista running slow on 1GB ram, but it runs perfectly fine on a laptop with 2GB ram, 2GHz dualcore and integrated intel graphics. I don't game much on that machine, though.
Third, OllyDbg is a debugger. It lets you handle program execution instruction by instruction. It works on individual processes, though, and what you want sounds like systemwide action.