problem setting up a dual boot Win7 & XP system

The situation is a little complicated (to me that is). Have Windows 7 installed on the C drive partition of a large (1 TB) HD. On the other D partition is Win XP. The problem is that the people who built the computer used a bogus copy of XP to set it up. I have a real one and want to replace the bogus one. I thought I could change the boot order in the BIOS and have the computer boot to the MSFT XP disc. But after I change the boot order to the DVD drive it still boots to the screen with the choice of Windows 7 or XP. If I choose XP, I am in WinXP and can't get the CD to overwrite the bogus XP. I do not want to chance anything happening to my Windows 7 installation as it's perfect and has a lot of work in building all the programs etc.
Any advice or direction to look into would be appreciated.