If you are having problems with getting your MA790XT-UD4P motherboard working in windows 2008 with Hyper-v enable this is how you fix the problem:
1. Bios revision F4 and F4G currently do not have the correct patch to allow Phenom II processors to work within hyper-v
2. What you need to do is apply Bios f3 9an older revision which has the patch
and you are good to go