Here are some things to try:
Empty the Recyle bin if you haven't already.
To delete files and bypass the Recyle bin press and hold the Shift key and then press the Delete key (Shift+Delete).
Try running Disk Cleanup by doing the following:
1. Click on Start
2. Click on All Programs
3. Click on Accessories
4. Click on the System Tools folder
5. Click on Disk Cleanup
6. Click on OK