As a reminder, we do not allow people to request/offer help with piracy or otherwise promote/gloat about it. I've moved a couple of posts from here that were breaking this rule; please do not repeat it.
-ViRGE
OP, if you don't wish to pay for Windows, then as the others have noted Linux is your best (and really only practical) option. However it's not a very good gaming OS as relatively few games are available for Linux, and the WINE environment is limited at best. So really you're going to want Windows.
And considering the money you're going to be shelling out for a new gaming computer, $100 for an OS that can last you nearly a decade is not too much to ask.