cc-cleaner. Cleans up shed loads of files (doesn't just do registries).
Check add/remove programs and remove and crud you don't use.
I just checked my DAW app partition, I'm running a ton of stuff including Windows 7 and Komplete Ultimate. I am using just 130Gb. Data and VST's are on data partition (on a different drive) and that's just 300Gb. The system right now is really really clean, my backups are done to server.
I'm confused how large your hard drive actually is, also (just to say) there is a difference between a hard drive and a partition.
It might be time for a second hard drive and consider a reinstall of Windows, separating your applications from your data (I do this sort of stuff for fun, mad I know)