The winsxs folder in c:\windows can be really big, the only way to cleanup is using the microsoft command line tools :

Windows Server 2003 : VSP1CLEAN.exe
Windows Server 2008 : COMPCLN.exe
Windows 7 and Server 2008 R2 : DISM.exe

I will explain the last one here.  Start your command prompt As Administrator.

Usage: dism /online /cleanup-image /spsuperseded

Note:
After running this command it won’t be possible to uninstall any service packs!

Before :

After:

So almost 3Gb on my Windows 7 Sp1 machine is released.

You can also run these commands :

Takeown /f %windir%\winsxs\ManifestCache\*
Icacls %windir%\winsxs\ManifestCache\* /GRANT administrators:F
Del /q %windir%\winsxs\ManifestCache\*

In my case this cleanup another 500Mb

Créditos : http://www.microsoftpro.nl/2011/07/28/how-to-cleanup-the-winsxs-folder-on-windows-7-sp1-windows-2008-windows-2008-r2-sp1/