Hello and welcome to the Vista Forums!
I have got quite a lot of say! First and foremost, that size is incorrect! The Windows Side by Side configuration stores a lot of duplicate files, exactly the same. However, the other copies are only hard links to the first copy, and don't take up any additional space. Those first copies may not even be in there anyway, and may point to somewhere else again!
Hard links are used to link these folders. These are far more robust than shortcuts or symbolic links. They show as the full size in Windows Explorer or WinDirStat. My point is that your file size is not 12GB. Microsoft always quote 400MB of actual data taking up space as manifests, first files, etc. etc. Your folder is quite large, so perhaps you are more around the 600MB line.
You can get some more general information here: Demystifying the WinSxS directory in Windows XP, Vista and Server 2003/2008 - Aaron Tiensivu's Blog
Sadly, you cannot delete any of it, and it is very difficult to shrink. The best you can do is run vsp1cln
from a Command Prompt to remove some pre-SP1 stuff. Note that you can now no longer uninstall SP1, although this shouldn't matter too much, and you cannot then get pre-SP1 hotfix corruption replacement files out of it either, although that should matter even less.
If you have any questions, do not hesitate to ask!