It's so bad, that after quitting this program, the time-slice quantums are adjusted by Windows XP to make all the other programs sluggish.
- is that actually what happens? (there is - or at least used to be - some quantum/timeslice config you could go at in the registry, but afaik that required a reboot, and I'm not sure the settings work any longer... and if a game messed with them, it would seem weird that logging off/on would restore if quitting the game didn't).
Differently privileged stuff on XP (and everything before it) was a headache, Vistas UAC is actually quite a godsend in that regard. You could probably create a non-privileged user and use "run as user", but it's been years since I messed with it (thankfully!) so the details are hazy. Other methods to drop privileges are flaky.
But are you sure it's a high vs. low privilege thing? Could it be something else, like the CPU/GPU overheating and causing thermal throttling - and that "logging out fixes it" is because of the time it takes doing the relog jig, rather than the act of relogging?
As for flash, it's a well-known hog, and I doubt there's much you can do about it in Firefox (apart from uninstalling
) - Chrome doesn't use the default Adobe plugin, they have their own tweaked version. If Process Tamer on plugin-container doesn't help, I dunno - my experience is that flash is generally a pretty sluggish hog.