First of all, does that version of Windows XP offer you a license to run on that other computer? Most versions of XP sold are OEM versions, and it's illegal to transfer them to another PC.
Then, apart from the legal aspects, there's the technical side. It's quite likely it won't run without doing a fresh install. Than you can format the disk and get rid of Ubuntu that way.
Assuming it's legal, and assuming it works, you would have to go into Ubuntu's bootmanager and tell it to default to XP in stead of asking.
I have a Dell Optiplex GX620 which is old but still works good for browsing etc. I hooked up a spare hard drive that BSOD on me the other day and would not take Windows XP when I tried to reload the OS. I was able to load ubuntu 10.10 to try it out though. Ubuntu worked fine and I could either boot into Windows or Ubuntu. When I took the Ubuntu drive out to put it into another older computer for the kids to play with that is when I noticed that it was asking me whether or not I wanted to boot into Windows or Ubuntu. Since I only want Windows on that computer now, how do I get the boot screen to stop asking about which OS to boot into?