Wow--- Hold the Fort !!!
I have set up and can play all the games (at least games released after quake 1) with no problems on a PII 350 machine.
From Quake II on works fine.
Hell, I can even play Max Payne-- even though is is below the 500 specs minimum.
So... the problem is not the games or XP itself. Initially I did an upgrade from W98 but had to try to fix too many quirks-- so I did a complete reformat and clean install of XP, upgraded my video driver (the driver supplied by XP sucked) then reinstalled and patched the games.
So far, other than the mouse problem with Half-Life/Counterstrike which is easily fixed, the games run fine.
Oh yeah-- with the low screen refresh rate you will get a headache after a while, but what the hell-- small price to pay!😀