1. Run a hard drive diagnostic.
Take a look under the hood and see what make it is.....they all have their own free diagnostic utilities.
I believe that they are all on this free diagnostic disk...
http://www.ultimatebootcd.com/

2. If the HDD is declared physically OK after the diagnostic...
Some Dells have a "hidden" recovery partition. You'll have to Google around for the procedure.
It's there to recover the operating system after a fatal crash and restores the system to an "as shipped" condition. All personal data is lost, which doesn't matter in this case.
You can also ask the woman if she has any of the disks that came with this Dell.
(having these will simplify everything)

3. There should be a product key taped to the bottom of the lappy.
All you need to do is borrow an XP disk and input that product key at the prompt during the install.
Note:
The XP disk must be the same variant as the original, for example...
Chances are greatest that this machine was loaded with an OEM copy of XP Home.
The product key will identify variant and it will be an OEM copy (as opposed to Retail)

4. Cleanup.
After you get up and running with a generic XP disk you are also running on generic drivers.
Install security software and update drivers.
Recommend custom install (turn off what you don't need) of AntiVir for the best freebie AV solution.