I don't know a good war game for that speed computer but Prairie Dog Hunt Pro (Will open in window.) is a free download. Supposedly this is shareware, but I can't find a register button, it does not have any 'crippled' features, and it hasn't come up with a nag screen after an hour or so play.
I've blasted quite a few virtual prairie poodles with it. Lots of fun, should run on an older machine.