First the best thing you can do to free up some disk space is to run Disk Cleanup:

My Computer >> right click on C: drive >> Disk Cleanup

Also, you can defragment your drive which should make all your files whole and compress a lot of them.

One thing I suggest which could take all day (though it usually doesn't) depending on your HD size/CPU speed ratio, is compressing your drive.

My Computer >> right click on C: drive >> Check the compress hard drive box >> OK.

This should free up a few gigabytes of space at most.