You can't delete this folder. It's the assembly cache.
From TFA:
The Winsxs folder, stores multiple copies of dll's in order to let multiple applications run in Windows without any compatibility problem.
Link is: http://www.winvistaclub.com/f16.html
EDIT: compcln.exe, part of Vista SP2, allows you to remove RTM and SP1 files from the cache, but it will not allow you to roll back to a previous version if you wanted to. The Service Pack Clean-up tool is a replacement of Vsp1cln.exe from Vista SP1 that did much the same thing. See more here.