XPS / Dimension One - XP licence / Install issue

I'm looking to install XP on the Dimension One, but I'm having issues when using a Dell XP disk as it won't activate. The code given then won't "work" when using the microsoft phone line.

I thought a Dell XP disk would / should work. Shouldn't a Dell disk just install (which happens) AND activate?

I have noticed when in the BIOS, a Service Tag doesn't show and wondered if this has anything to do with it. Should I upgrade the BIOS?

Although there are no XP drivers on the site I have seen threads where users have installed XP without issues.

My question is not so much about that, but just being able to correctly and easily activate the product with XP. I have three of them to do for a client. I have the Vista Ultimate disks and an XP Pro disk (although not sure this desk came with the system or is from something else as client handed over all their disks!).

Do I contact Microsoft, Dell , both or neither?! Any help much appreciated,

Cheers,

Matt