Installed Windows 7 Ultimate, but also need XP

New user here. Just installed Parallels 7 on a MacBook Pro running Snow Leopard. I've purchased and installed the 64 bit version of Windows 7 Ultimate. I believe I will also need to be able to run Win XP from time to time, though. So, my question is how to be able to run WinXP? Do I run it as it's own virtual machine within Win7 (running under the Parallels virtual machine), or can I run it directly under Parallels (assuming I can find a copy somewhere).
I've downloaded the Windows Virtual Machine and Windows XP Mode software from the Microsoft site www.windows.com/downloads/business, along with the Windows XP Mode update. When I try to install Windows XP this way, the progress bar gets right to the end, and then fails. This has happened twice.

So how to get XP running? Thanks in advance.