search (here in LQ you will find many many manu docs if you just do a search) a thread that concerns something like dual-boot and/or disk partitioning and/or bootloaders. there you'll find the exact instructions...it's a waste of time to write them in every thread
do a search; quick steps are
1) partition your HD so that you have space for linux
2) install linux
3) configure bootloader and bootplace (boot partition/MBR) so that some linux's bootloader overrides windows' own and so that it will recognize both OS's
but still, if you only need linux for that purpose...why not download a live-cd like Knoppix and use it?
I don't get installing an OS to keep on playing one friggin game. especially if it _can_ be done with the existing partition...
oh, one more thing - do hackers that do bad things, really care about "rules"? in a game? what?
EDIT: well, if you really are going to install linux, why not wipe out windows in the same run?