Dual boot Vista with XP

I am not a computer expert, far from it, and do not now a lot about it but I am sure this problem I am experiencing at the moment is something that raises anyones blood pressure. I am an elderly enthusiast and love to spend my spare time on the PC. But now I am seriously thinking of giving it away. This is my problem:
How to install Win Xp on new PCwith Vista OEM installed on Acer Aspire E700. When trying to install the warning came up:This program is blocked due to compability issues. Settup cannot continue because the version of Windows on your computer is newer than the one on the CD.
Warning, if you decide to delete the newer version of Windows that is currently installed on your computer, the files and settings cannot be recovered.
After spending a lot of money on a new PC I am very frustrated that I cannot choose what to do with it. I do have my own Win XP CD but I will not buy another Vista Cd just to install it the new version. I hope there is some one out there who can point me in the right direction to install Win XP on an other partition without loosing Vista.
All advise is truly appreciated.
Johanna