Microsoft really doesn't want people messing anything in that folder without using official tools (I assume, one of the checkmarks in windows built-in disk-cleanup tool touches this folder)
Here's a good article for you to read (especially the updates near the bottom of the article)
Hello trium - NEVER delete any of the files in the WinSxS folder manually!!! You could do some very serious damage to your system files and probably have to re-install Windows as a result.
A much safer aproach is to use Windows Disk Cleanup and select the option to "Cleanup System Files". After it has finished scanning and has generated a list of items to delete, go through the list and find the line that's related to Windows Update files. Doing this will clean out any old updates that have been superceded by newer updates.