So this is what I've been doing today:
1. Start a backup of the laptop.
2. Wait for ages until the backup completes.
3. Boot from the XP CD.
4. Re-partition the hard disk.
5. Format the startup partition.
6. Allow the XP installation to continue.
7. Watch it re-try a read on the CD.
8. Repeat #7, several times.
9. Find a spare hard disk and a spare PC, and repeat steps #3 to #8.
10. Look at the big scratch on the underside of the XP CD.
11. Hunt around for, and fail to find, a spare XP CD.
12. Start a restore from the earlier backup.
13. Wait ages for the restore to complete.
Not quite what I'd planned today.