Well, I don't have any experience with Apples airport - but, gonna scratch something hopefully.
If you have any security enabled on your Airport - whether it be MAC Address Filtering, or encryption (or both) - Be sure you have the settings correct within XP. It might not be assigning an IP address to your XP install, because XP is not providing the key your airport is looking for.
I'd log into OSX and look into your airport settings on the airport - see if you have any encryption key, and temporarily disable it (make your network open). Write this key down, so you can type it into windows TWICE.

I can't imagine MAC filtering being an issue, because I believe the MAC address is on the hardware itself - so that shouldn't change regardless of what OS you're running. (as long as it's on the same computer) The only thing I see changing would be that encryption key.
Usually, setting up a network, I plug the computer into the ethernet cable going to the router, set it up, activate wireless, and check to see if it's getting an IP assigned before disconnecting and going straight to wireless.