System folders which you can empty to save you diskspace:
C
WINDOWS\Temp
C
WINDOWS\Prefetch
C
WINDOWS\Downloaded Installations
C
WINDOWS\system32\dllcache
C
WINDOWS\Installer (in this case you have to check which installers you don't need and then delete them)
C
WINDOWS\Help (this solution is optionally, if you don't need system help you can follow this tip)
C
Documents and Settings\(*your username*)\Local Settings\Temp
C
Documents and Settings\(*your username*)\Local Settings\Temporary Internet Files