Ultra VNC can be run as reverse vnc and will even build a little self-contained executable that is preconfigured. Install the remote on her end and all she will need to do is doubleclick. You can point it at a ddns resolver if you don't have a static IP. There is also the old school remote assistant built into windows which works ok.
That said, I would suggest moving her from xp to windows 7. It is much more secure and you can change most things to "classic" mode to make them look XP like to make learning easier.
You can also install a router capable of running clamav or some sort of scanning to check those incoming wallpaper executables etc. for trojans and that might make life a little easier.
Oh, and if hardware upgrades are ever on the map, consider a mirrored raid. Harddrives for home use are cheap and most motherboards support raid1 nowadays, so considering how few people actually make backups this might be a lifesaver for her and one less headache for you too. It won't protect from everything a good backup will, but it will save her from a sudden disk failure and the built in backup on windows 7 is sufficient for many home users.