Windows Server 2008 Hyper-V RC0 and Athlon 64 X2 4200+

I used virtualization through VirtualPC 2007 on this computer, an Athlon 64 X2 4200+ with a Gigabyte GA-M55SLI-S4 motherboard, and it worked perfectly. The Athlon 64 X2 even support Hardware-Assisted Virtualization, so things actually run pretty smooth. Now I've upgraded to Windows Server 2008 so I can run virtual machines with the new Hyper-V (RC0), but when I try to start any virtual machine it gives me an error saying "The virtual machine could not be started because the hypervisor is not running." What?? I just installed it. Did I miss configuring something?