Windows XP is seeing a dual installation. Try to install your XP CD and instead of installing go to repair...tools...locate boot ini....see if you can remove the XP reference.
I installed Vista on a HP Pavilion PC with no issues whatsoever. I chose to install over the existing XP installation as opposed to a clean install. However, when I boot up the PC now asks me to choose between booting from a previous installation of Windows or Windows Vista. Preferably I would rather it just boot from Vista without asking me first. I chose the "previous instalation" option once just to see what happens and it just takes me to the recovery menu...
If anyone has any suggestions on how to address this problem I'd appreciate it.
Thanks.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic