Adding my little chunk o' experience, multiple core systems seem to have a problem with the task switcher under XP. Switching out of a task running in user space appears to occasionally result in problems with the task when switching back. I know for a fact that you tab in and out of programs willy-nilly, so this might be a possibility. Do they *always* run choppy from the very moment you load them, or only after you've switched out of them?

I have my suspicion that, since the alt-tab function is implemented by CSRSS, which also handles threading, the problem lies in CSRSS trying to reassign threads on a core (just my guess, not in any way diagnostic). If that were actually the case I'd expect setting the affinity of the game to a single core to remedy the situation.