Reduce the size of WINSXS folder

1. Click Start | Run
2. Type in CMD and press [enter]
3. At the command prompt, type DISM /online /Cleanup-Image /SpSuperseded and press [enter]

While you trying to do it, if some Error is shown than your OS was probably installed using media that already had SP1. In this case please use the Disk Cleanup tool:

1. Open My Computer,
2. Right Click on the Windows System Drive and click on Properties, 
3. Then from the Properties dialog click on "Disk Cleanup" in General tab.
4. Wait for a while to open the "Disk Cleanup" dialog.
5. Here it has an option on lower left "Clean up system files". Click on it.
6. "Disk Cleanup" dialog will re-open.
7. Now you will find "Service File Backup Files" in "Files to delete" box. Select it and click Ok.
8. A conformation pop-up will open,
9. Click "Delete Files" to Proceed.
10. Done

Vista

It depends on which SP your OS is running:

For SP1 use vsp1cln.exe
For SP2 use compcln.exe

Both tools are located in %windir%\system32