If it had Vista OS on it, it probably has good enough hardware to handle Windows 7; so why not move up instead of moving down. XP is no longer marketed, its patch development teams curtailed, so i tis more vulnerable to viral and spam attacks. To move up, you need:

(1) download the current bios from manufacturer and check to see if they have included a signature (slic certificate) for windows 7 (this usually happens to the machines that were sold with Vista already installed on them); if the manufacturer has not included slic 2.1 certificate in their current version of bios, you go ahead and insert it.

(2) find a certificate for windows 7 available from the same manufacturer that made your computer (it doesn't have to be for the same model you have but any certificate issued by the same manufacturer would do)

(3) find an OEM (original equipment manufacturer) serial No for the particular version of windows 7 and use it to activate your win 7 locally. After this, go out and get the updates...

You should be set.