6 months ago i had to reinstall Windows. I couldn't reformat C: before doing that.
I still have an old folder C:\Documents and Settings\John last accessed in September 2013, created in 2008 which I cannot access as administrator. It clearly has quite a lot of content and appears to be taking up several gigabytes of space. (SAV takes several minutes to scan its contents.) My new user name is JZ.
(Why can SAV scan it but I can't see it even when viewing of hidden and system files is enabled?)
There's other old stuff there as well in C:\Program Files but I don't think the space issue is as important for them.
By the way, do you approve of the advice that the contents of C:\i386 can be moved to another drive? See for example http://ask-leo.com/can_i_move_or_delete_my_i386_directory_to_free_up_some_space.html