Okay, my C drive was getting a bit full and my programs started to run a bit slow, so I decided to clean up a bit and defrag. The cleanup added a few more GB's to my free space, but nothing really significant. The defrag didn't really do anything significant either, but it did give me a list of errors in compressing and moving files. I went on a bit of a hunt, finding all this stuff, and discovered that most of it was stuff that I got rid of last year! Basically, files I deleted using programs, like iTunes movies and WMC recorded TV shows and such, WERE NOT DELETED. The files I found were eating up over 40 GB of space! Now that I got rid of all that poo poo, I have about 70 GB. Now I finally don't have to worry about having enough space when installing new games and programs :D

A word of wisdom: if you have to delete a file, delete it from explorer, not whatever program you were using like iTunes. Also, if you're HDD seems oddly full, check it thoroughly.