I have a Vista Home Premium pc SP1
The Hard drive is half filled with multiple folders.
Under one user under \local settings is an application data folder that contains many other application data folders nested in it, many many times.
That same user under AppData has the same thing - an application data folder that contains another application data folder and inside that another each one a a couple of gigs smaller than the last.
Over half of the hard drive space seems to be used up by these nested folders.