If you’ve installed service pack 1 for Windows 7 or Windows Server 2008 R2 and you’re happy you won’t need to uninstall it, you can cleanup the service pack backup files by running the command below from an elevated command prompt.  This uses the Deployment Image Servicing and Management (DISM) tool

DISM.exe /online /Cleanup-Image /spsuperseded

The command freed 1.6GB on my Windows 7 desktop and 1.8GB on a Windows Server 2008 R2 Std server.

You can do the same for Windows Vista with vsp1cln.exe for service pack 1 and compcln.exe for service pack 2.