I think one key part of the solution hasn’t been posted here: set a password on your screen saver, and make sure it cuts in!
After running a computer lab for 240 people (not all at the same time), I’ve learned that an ounce of prevention beats a pound and a half of cure. Make sure you’re not using your administrator account for general stuff, particularly on XP, use Firefox if you don’t already, and perhaps drop some hints that you’re really not interested in 40 Free Smiley Mouse Cursors. The Billfred technique of intimidating them into treating them nice because word gets out as to who messed the computers up may or may not be particularly effective for you.
Alternatively, burn a Knoppix CD for the next time she comes over, and have her work on that. It’ll do the job, and you just eject the CD and reboot when you’re done.