On Windows Server 2008 R2 the folder winsxs (generally under c:\windows\winsxs) can grow bigger and bigger through time, saving all system updates.
To remove superseded backup images, you can use the dism command from the Administrator command prompt, with the following syntax:
dism /online /Cleanup-Image /spsuperseded
Be patient, it can take up to 10 minutes and recover up to 5 GB from the winsxs folder.