Say_my_name
Warlord
Sounds like Serious Sam is exactly what you want. It is awesome. Hordes of dudes come at you and you shoot them. And I do mean hordes of dudes.
Painkiller is a slightly newer version of pretty much the same thing except with more of a gothic horror theme happening. Also there is a gun that fires wooden stakes that you can nail dudes to the wall with. It is also awesome.
They're by far the most fun shooting games of the last decade, will easily run on an older computer, and have exactly nothing to do with wwii
EDIT: Painkiller: Black Edition is $5 on Steam right now. You should get it (ignore the sequels though).
Yes. Painkiller is a masterpiece of mindless slaughter. The leveldesign is beautiful, the action is fastpaced and both the enemy and weapon design are creative. And ignore the sequels, they are horrible.