After a successful installation of Windows 7 SP1 you may want to save disk space deleting the backup files created by SP1 installer. After that you cannot uninstall SP1 from your computer.
From an elevated command prompt enter this command:
dism.exe /online /Cleanup-Image /spsuperseded
For more info read the Deployment Guide for Windows Server 2008 R2 with SP1 and Windows 7 with SP1