Hi Ello.
Do you have more then one partition? You can format other partitions by going to "my computer" and rightclicking the drive and select format.
But you cant format the partition with the O.S. on it without removing the O.S. But what you could do is uninstall/delete everything that you dont want on there , and then use an app like Eraser to "erase" the empty space around the O.S. so to say.
Install the app , in "my computer" rightclick the C: partition and select "erase unused space" Does alot better job then the normal format on the Guttman setting , lol.
http://www.heidi.ie/eraser/download.php