Hardware Virtualization on Optiplex 755

Hola Amigos!

last week I received my brand new PC (Dell OptiPlex 755 with E8400 Intel
Core 2 Duo), and I'm not able to get Hardware-assisted Virtualization running
on Virtual PC 2007 SP1.
My host system is a Vista Business x64 with 4 GB RAM, and all Virtualization
options are enabled in the BIOS. I even did a BIOS Firmware update to the
latest - no change. I read, that sometimes you need to do a cold boot in
order to make Virtualization changes in the BIOS work -> no change.

Am I missing something? What else could be the reason for VPC not to allow
me doing hardware virtualization? I found a tool on the internet
http://www.grc.com/securable.htm that I ran on my PC, and the result was "64,
Yes, Yes" which means that my 64-bit OS is able to do hardware virtualization.

I even asked Microsoft for their help, and they said, that it could be a bug in the Dell BIOS. Any idea if this is true?

Thanks for your help guys!

Cheers,

Thomas