You don't tell us the size of the hard drive, so we could only offer general advice, but uninstall programs you don't use any more, remove all but the latest System Restore point, and use a utility like CCleaner to clean out all unwanted Temp folder files.
Move large files, like video and music files to some other storage medium.
Mark
My C drive is always running out of space,What can I do to fix this?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic