Well, if she erases the hard drive, the PC won't be worth much to a poor family that can't afford to buy the XP OS.
She can however, delete her personal files and use this program to erase the unused space. This free space contains the deleted files, most of which can be recovered unless their space is written over.
http://sourceforge.net/projects/eraser/
While it's capable of a secure erase, that will take a long time, maybe days. Set it up to write a single pattern once, and it's fast and will defeat all but the CSI types with specialized hardware.
If she has passwords stored in a browser like firefox, it will take some knowledge to clear them. Probably best to change all passwords.