If it is a new PC, I think it would be safe to delete the "OEM" folder, but back it up in case there are driver installation files which you may want to use later on.
If you upgraded, I think there is "Windows.old" folder. Delete that. It may not let you, so just "Take ownership" of the folder
http://www.howtogeek.com/howto/windows-vista/add-take-ownership-to-explorer-right-click-menu-in-vista/
Also $Windows~WS$ and $Windows~BS$ should be safe to delete.
Otherwise, delete all the files in:
C:\Windows\Temp
and
C:\Users\Username\AppData\Local\Temp