Yep, you'd need not only the XP disk but one that matches the type and version of the license key on the sticker. (chances are you'd need an OEM version of XP home)
The CDs aren't supplied with a lot of machines now, you are meant to burn the recovery disks yourself when you first get the machine (which you/her would have got prompted to do until it was either done, or somebody selected the don't prompt me to do it again option)
If you can borrow an appropriate XP cd (know anyone with a dell computer that came with XP home? their CD would probably work) then you should be able to use that fine.... otherwise you'll probably have to buy XP (or if it's a half decent laptop you could upgrade it to windows7)...
Otherwise.... sticking one of the versions of linux on it would make it usable again for most things for free, but this is often not a suitable thing to do to people that are totally used to using windows..