You can use the Disk Cleanup tool located in All Programs, Accessories, System Tools. This will scan your HDD for all files that can be safetly deleted, as well as compress old files to save on disk space. You'll be amazed how much disk space you can free up if you've never run this before. I've seen up to 10 GB freed up (combined total of Recycling Bin, Setup Files, and Temporary Internet Files). This doesn't delete cookies, either. It assumes that you want to save those.
_________________________
It's most likely a problem with your flux capacitor.