System Freezes on Shut down / Reboot

I have Vista Beta 2 (build 5384) x64 installed on my machine and whenever I
shut down or reboot, the system locks up at a black screen. The shut down
process appears to be normal with the operating system informing me that it
is shutting down, etc. However, when the screen goes black and where I would
expect the computer to power off, it doesn't.

I am not going into hiberation mode. I cannot CTRL-ALT-DEL, I cannot turn
the computer off using the power buttom - I must do a hard reset.

Any ideas? Thanks.

ASUS A8N-VM mobo with AMD64 3700 and 2GB RAM.