Ok, the definitive answer is in here: http://www.winvistaclub.com/f16.html
.. which leans towards "don't touch it !".
What is going on is that when u run an upd8 for Vista, not just a service pack, any of the files within the original Vista install, mainly the likes of dlls, are linked to and put in Winsxs.
eg. If u run an up8 that includes idontgiveanf.dll (version 2.0), version 1.0 gets copied to Winsxs with an XML reference to it's existance.
oooo xml again.
Now unfortunately there can be well in excess of 25,000 files in this folder, within subfolders, including reference files and the xmls. All ofc adding up to anything around 15g of data. Nice. This is ofc dependant on how much upd8ing u've done. Or I guess what level of sp ur original Vista is - AND if u have Ultimate or a lesser requirment version.
Why do it ?
They've done it so that if they upd8d idontgiveanf.dll to version 2.0, and u have a util that relies on idontgiveanf.dll version 1.0 - the util won't panic.
This is crazy talk imo, but that's the size of it.
"We'll upd8 the files, and keep the older ones incase something needs them."
This is data duplication of a gigantic scale for the sake of it. What is almost the case here is that they're saying idontgiveanf.dll (version 2.0) won't contain ALL the libraries of version 1.0 (which in some cases, like c programming, u find is the case), and just stick with the new stuff. Now this is all well and good IF u don't keep hold of the old version. U purposely don't include the older versions libraries to... save space.. otherwize u would just add the new library stuff to the old one and bump the revision (which I believe has been practice over the years).
So why make a point of removing the older versions from the new versions - but keep the older versions anyway ?
Don't know tbh. It's both illogical, bordering on stupid. U may aswell have just tacked the new on the old.
Now - there are differing reports on what u can do about this. Pushing 30g for an OS install, over half of it being in a location like this is madness.
Some people have deleted all it's contents with success. The only downside I've read about to this point is utils/progs not working cos they look for the older file versions and they aint where they're reported to be. Therein u have instability, crashing, etc etc. But if u have nothing that might do this ? hmm I dunno. I may well experiment at some point by just renaming the folder. If I get issues ofc I can stick it back - or if the same file keeps coming up (if there's a way to find out) just put it back on it's own. Dunno how easy it would be to do that dependant on how many complaints u get.
(and ofc uninstalling upd8s..... well how often do we do that ? u'd have to decide)
It should be the case that if u buy SP2 of Vista, u won't have this problem - YET. And if u use SP1, u'll have less of a problem. Which kinda makes me feel the deletion of the contents should be semi ok. I'll see.
What a big cluster-F if I'm honest. Surprized ? No.
(be warned, u delete stuff at ur own risk, I aint saying to do anything)