Which one?
3D LOLOLOL. (it's still awesome)
Try Return to Castle Wolfenstein, it runs on a 12-year old engine. Though he's probably talking about the lame CoD clone new Wolfenstein.
If you can run Modern Borefare 2 on it, you should be able to run Source engine games just fine.
There's always Doom. Doom is still, and forever will be, awesome. And also better than CoD. Seriously.