Your page file may be growing. It's size can be set.http://tweakhound.com/windows7/tweaking/7.html
Restore points also take up space. You should be able to delete all but the last restore point via "disk cleanup" on the advanced tab.
Windirstat is useful for finding large files and folders.http://windirstat.info/
Also, if you don't use hibernation, go into the power settings and disable hibernation, then delete the hyberfil.sys file.http://www.sevenforums.com/tutorials/819-hibernate-enable-disable.html