I guarantee that the original Doom game will work, if it runs smoothly on my iPod Nano, it will work on his PC. Plus it was made in 1993.
Games that will probably work according to the specs posted on Gamespot and/or Wikipedia:
Doom
Half-Life
Quake
Starcraft
Unreal Tournament
Command + Conquer: Red Alert
I'm sure there's plenty more, just can't think of anything else right now.