I presume she just doesn't want anyone to have her personal files.
I just cleaned up a machine in my office that I was leaving to the next user. I deleted all my word processing files, and anything else that was personal. I'm not paranoid, so I didn't use one of those programs that scrubs deleted files so they can't be recovered, but it can be done.
The only problem with that is that you have to know what's on your machine. It would be hard for an outsider to do the cleaning up.
If she has all the windows installation disks, the alternative would be to FORMAT C: and reinstall the operating system. But I always hesitate to do that unless I'm sure I have everything I need, such as the windows disk cover that has the Windows ID code on it. Without that, you can't reinstall Win98 unless you're a hacker.
You have to go find all the IE caches, Registry entries, etc. which are hidden and delete them. I've bought all sorts of used hard drives that still had personal stuff on them even though the obvious files had been deleted. In addition one has to run a program to write over the deleted files to make them unrecoverable.