If we were handling it on our tech support lines, the first thing to do would be to put the original OS back using the restore disks - simply because it is the quickest way to find out if there's a HW problem or not. We don't support anything other than the original OS/installation for reasons just like this: until you have the original configuration back you never know what is going on.
Does the HW show up in Device Manager under 'other devices' or anything like that?
Why did you put XP Pro on there anyway? Was there some specific feature you wanted? We get a lot of people doing this simply because they think Pro is better than Home or MCE, but they don't actually need it.
It is also my understanding that MCE is essentially Pro anyway.