A couple points here:
1. Go to the Dell support site and make sure there are XP drivers available for that model Dell. Had a customer just a couple weeks ago with a Dell Studio laptop and wanted to install XP for nearly the same reason... he just "liked it better". Luckily for me, I checked with Dell first and discovered that the XP "Downgrade" option was not available on that model, and there were zero XP drivers available. You might be able to scout around for various hardware drivers for each individual component, but it would be very time-consuming and frustrating. If that is the case here, I'd recommend explaining the situation to the client and do your best to convince him to stick with the Vista he's aready got and licensed for. Which leads me to my next point:
2. When your customer bought the Dell, he also purchased a license for Vista and Vista only. For Dells that offer the "XP Downgrade" option, you are actually purchasing two licenses; one for Vista, and one for XP. Hence the $99 charge for the "downgrade". If you had a Dell XP Pro SP2/SP3 CD, it would install (though you'd still potentially have the driver issues I mentioned above), but it wouldn't be legal unless you also purchased a licensed copy of XP for the client.