I've installed Microsoft's Virtual PC on Windows 7, but when I try to start a virtual machine I get the following error message:

'<Virtual machine name>' could not be started because hardware-assisted virtualization is disabled. Please enable hardware virtualization in the BIOS settings and try again. If hardware virtualization settings is already enabled, you may have to disable Trusted Execution Technology (TXT) setting in BIOS or update the system BIOS.

However, if I download and run the Hardware-Assisted Virtualization Detection Tool, it gives the following positive message:

This computer is configured with hardware-assisted virtualization.

This computer meets the processor requirements to run Windows Virtual PC. If this computer runs a supported edition of Windows® 7, you can install Windows Virtual PC.

I've also used the MSR Walker in the third-party utility CrystalCPUID to examine MSR 0x3a on both processors on my system, and it's 0x5 (0x4 = VT enabled, 0x1 = VT lock), as expected.

Does anyone have any ideas of what else to check? Thanks.