- Make sure the "Desktop Experience" is installed.
(Note that if you need to install it, it requires a restart and it takes some time to install on the next boot)
- Run "Disk Cleanup" (cleanmgr.exe)
- Click on "Clean up system files"
- Select "Service Pack Backup Files"
- Click "OK" to start the cleaning (it can take quite some time, but will give you ~ 1-2 GBs)