Already used "Ccleaner"? It removes lots of temporarily Internet files, and other stuff. Assuming you watch lots of p0r... Love video's it's very useful to remove your temp files. I don't know how much you play different gamemodes, and downloading stuff from garrysmod.org. but that would fill up your HDD very much to.
Try to delete all of your old music files, and old games (U can always re-download them).
Set your music folder into details, click the date tab. remove your oldest music.
Let's be honest, you don't listen to all of them right? O_O
Still not useful information, then i give up.
Finniespin