Just came across this quickly and thought I would spread the word. If you are trying to free up disk space on Windows Vista or 7, check the Windows/winsxs folder. Apparently windows stores old copies of system files here in case you ever need to roll back a service pack. If your system has been stable for a while and you don’t anticipate this, you can safely remove some of the old files. Windows even provides a tool to do it for you.
Just open a command prompt and enter C:\Windows\System32\compcln.exe then hit enter. This is the Windows Component Clean Up Tool. It will remove unused old copies of component files (.dll etc) and free up some space. Mine went from 11.6GB to 9.33GB. If you want more technical details on exactly what is going out check these out:
Hope it helps!
*NOTE: If you do this you WILL NOT be able to roll back a service pack – fair warning.