Thanks for the replies.
1. Yes, the everyone is using exactly the same XML file. It is generated at run-time, but it is definitely the same each time for each user.
2. The machines are very similar, both running Win XP Pro with Service Pack 2.
3. Haven't been able to test it under a third person's account at this stage.
4. Yes it was a typo. It should have been b68y4N6E instead of b68y4N63.