TFP -
Vaios usually have their hard disks formatted into 3 partitions.
a recovery partition that you can't see from windows and is usually left un-tinkered with...
c: is the one that has windows, all the programs and "my documents" (plus pictures music et al) on it by default.
D: is usually an empty partition, available for file storage, but in my experience rarely used as the "average" laptop user barely realises it's there. Thus C: fills up, they run out of space and go and by an external drive from pc world for 100 notes!
My advice is to scour her laptop for photos and music (the two big offenders amongst the younger generation...) and move them in toto to d:
If she's using I-tunes then re-configure it to use the new d: folder as her default music directory. If you simply "move" (as opposed "copy") "my documents" and the sub folders to d: you should be given an option to make that folder the official my docs in it's new location. (I believe this came up in a thread recently, I would put down an exact description of the process but I'm at work and time is short, would you believe!) Someone help me out?
In short, anything that is big and accumulates over time, move to D:, and educate her to use it!