I just reread your last post Whiskas.
She should be able to format C and then start afresh.
She will need to shut the pc down after formatting C,start it again and go into BIOS as its booting up.
Set the BIOS to boot from CD ROM then save and exit BIOS.
Restart with XP in the CD and it shoud give options to format and partition the drive.
Get her to set it at around 8 or 10 gig on the C drive which she wants to install XP on to and the remainder as D drive.
If it assigns the letters differently or doesnt format the remainder that can be done later and the drive letters changed around in disk manager.
Hope this clarifies it a bit