Alan - FreeBSD and PC-BSD

I have an HP Compaq 6005 PC that comes with the AMD Phenom B55
processor. I have been trying to install FreeBSD and PC-BSD on it.
Unfortunately it just reboots. I've tried burning a DVD and a USB
stick. I know that the media is good because it boots just fine on
other systems.

I would say that it just isn't supported except for the fact that
FreeNAS was running on it for nearly a two years before I moved to
another system.

I would like to start working with BSD more but it's currently my only
spare system. Any ideas why FreeNAS would boot and work fine but plain
vanilla FreeBSD won't even boot? Any help would be appreciated.

Sincerely,

Alan B. Clements
Saskatoon, SK