If you have a copy of Win7, it's not very hard at all. - Edit 1
Before modification by Zalis at 31/08/2012 02:24:06 PM
I have a partition set up on my iMac, and I think I set it to use around 200GB of my TB hdd. I have a spare Steam account that I load über cheap games onto for co-op, though. Using much less space might make more sense for you, and you can choose the size of the partition in the Bootcamp Assistant. (in your Applications folder, under Utilities)