Good game for crap PC?

Hey, I want to play a game on my mom's crappy comp. >_< I can't even play GunZ here! The only online game this comp can handle is Urban Terror.
Can someone please tell me what a good online FPS is for a crappy PC?

I play Prototype, Crysis, CS:S and GunZ at my dad.

It's not a FPS, but you can try Lightning Warrior Raidy.

Doesn't take much to run Crossfire.
Or Battlefield Heros.