Windows 7 seems to keep a winsxs folder (C:\Windows\winsxs) and it tends to grow pretty big.
If you want to cleanup, open a CMD prompt as administrator and run following command:
C:\>dism /online /cleanup-image /spsuperseded
Deployment Image Servicing and Management tool
Image Version: 6.1.7600.16385
Removing backup files created during service pack installation.
Removing package Microsoft-Windows-UltimateEdition~31bf3856ad364e35~amd64~~6.1.7600.16385
Service Pack Cleanup operation completed.
The operation completed successfully.
>> I saved about 6 GB diskspace !