Instead of deleting everything, you can download a utility that will find her XP Key (the code), with that and another XP CD (presumably yours) she can reformat and start over that way (although she may need to obtain the proper drivers for her PC, presuably available from the vendor's site).
Alternatively, she can buy a brand new box for well under $400 (budget permitting)
My friend's computer is running horribly and I have tried everything that I know to fix the computer except deleting the hard drive and starting over. The problem is, she can't find her Windows CD or any of her startup CD's. Would it be safe to delete everything but the Windows folder? She is running Windows XP by the way.