I'm guessing you have one 1TB hard drive with everything on it? Ok, first, delete things that you just don't need anymore. Then, run a defrag. Secondly, run off to a local computer hardware store (Tigerdirect, CompUSA, Microcenter, or maybe even BestBuy) and get yourself another harddrive (1TB is good enough). It shouldn't cost you more that $80 bucks depending on the brand. Get that, install it into the computer (also pick up a SATA cable just in case you don't have an extra one), and move everything that's not Windows and Steam into that drive.