Generally, you can't boot Windows from an removable drive... without a lot of hardware requirements and complication. See:
http://www.microsoft.com/whdc/device/storage/usb-boot.mspx and
http://groups.google.com/group/microsoft.public.windowsxp.general/browse_thread/thread/f4ac8ea85dc06036/77780a632a20aa3e%2377780a632a20aa3eWhen you installed XP Pro on your machine, it created a secondary install in a new directory, hence your dual boot. Have you tried to boot into XP Home since you installed XP Pro? What is the specific error you get on that? I'd be inclined to going back to that OS install.
If you decide to stay on XP Pro and can't get the video/audo drivers installed... I'd first focus on the drivers for your network adapter and try to download service packs. If you have a network card in it, hardwire to your broadband and you'll have better luck.