Assuming you are running Win 7 SP1.
Click the Orb (Start button)
type cleanmgr.exe
Right click Run as administrator
Select C: Drive, OK
Scroll down (after the scan) and put a tick in the Service Pack Backup Files
Select any of the other options you may find that have lots of unwanted files.
The Service Pack Backup Files trims down the C: \Windows\winsxs folder that store the Windows Update Files. It also maintains a copy of any replaced Updated files that are no longer required by the operating system.
If you are not running Service Pack 1, then Google clean out winsxs windows 7 and then enjoy some interesting reading.
Anyway, it's worth a look.