Whats using all this space?

[bledd]

yup

tons of people have multiple gigs 'missing' in SR and the shadowstorage

 

[TheVoice]

System restore, hibernation and page file most probably.

Indeed, the Hibernate file will take up a lot of space (the same as the amount of system RAM, in fact), and Vista tends to have Hibernate enabled by default. Bit useless for desktop PCs, so that can save a good few GB.