at this forum discussion about the winsxs folder;
http://forums.cnet.com/5208-19411_102-0.html?messageID=3069825#3069825
It is present in Vista and Windows 7.
If yours is growing that might be the reason, but as far as I am aware, there is no solution yet.
Also, have a look at Vista's System Volume Information folder, it is a hidden and protected System folder so you would need to unhide and un-protect in Folder Options. It stores System Restore backup files and indexing data. If the computer is otherwise running well you can delete all but the latest System Restore point in Vista's Disk Cleanup utility, or you can turn SR off, then back on;
http://techrepublic.com.com/5208-6230-0.html?forumID=101&threadID=227398&messageID=2264217
Mark
I have several computers running XP and needed a cheap media server and file share so I decided to take advantage of Vista's simple sharing capability. Unfortunately, shortly after installing Vista I noticed that my Vista continues to consume more and more hard drive. I started with a few gigabytes of Vista and around 20 GB of media and small office files which have now grown to over 150 GB and only 70 GB of that is media that I have actually added. I have temp file removing software installed on my XP machines to keep Windows small and manageable but cannot find one for Vista. Is my ever-growing Vista a temp file build-up from Vista's constant need to update and is there a tool to remove them? OR could this be something else entirely going on with my Windows "Virus" Home Premium?
Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic