How large is your C drive? One thing you can do is remove the temporary internet files.
Open My Computer.
Right-click the system drive, and then select Properties.
On the General tab, click Disk Cleanup.
Scroll down in the Files To Delete list, and then select Temporary Files.
sometimes this will get back a good portion.
Then you may want to remove some programs or games you may not have used or played in a log time. Some of these can take up a few gig of space each.
Lastly go to your file explore and open your C drive and sort it by size. This will show where mos of you disk is being used. If there is no size listed, like in a folder, double click the folder name or right click the folder to see the total size of disk being used by that folder.