Windows XP mode could not be restored because of either host processor mismatch or lack of hardware assisted virtualization support in the system.

That is the message I got when I start XP mode on a W7 system… after I made a huge number of changes to a system (while fault-finding).

Since I had updated the BIOS, then that was the first place to look.

Sure enough, the Hardware Virtualization had been switched off during the update.

I switch it back on, restart the PC, and promptly got the same error!

OK a search on the net quickly shows that the hibernation file can get corrupted (which is possible after all the work I had done on this system).

so I delete:

C:Users{User ID}AppDataLocalMicrosoftWindows Virtual PCVirtual MachinesWindows XP Mode.vsv

But I still get the error.

I go back into the BIOS, and while looking the the VT option, I notice a little message on the right side of the screen:

You must power down the system for settings to take effect.

What! I’d never seen that before.

It seems strange to actually power off the system (rather than just do a reset)… but it actually worked!