|Your issue may not just be an OS one. It could also be a hardware one.|
You can't legally download any OS. You would need to own XP.
What you would do in this situation, bit of a learning curve, would be to virtualize that version of xp.
You can get free downloads of vmware or microsoft virtual pc. This software allows you to "host" another OS from within Vista.
This would be the path to follow.
Google workstation virtualization or virtual pc to vmware for more information including how tos and detailed explainations.