Revert back to Home Premium from Professional

You can still run XP Mode on systems that don't support hardware virtualization.....

Download and run the Hardware-Assisted Virtualization Detection Tool

• If you receive the message “This computer does not have hardware-assisted virtualization,” your computer does not meet the hardware requirement for Windows Virtual PC, but you can still use Windows 7 XP Mode by running this hotfix. Be aware that XP Mode will run much better if hardware-assisted virtualization can be enabled.

More info here - Windows Virtual PC: Configure BIOS

How well it works??? I can't say.

OP is attempting to change versions as a last resort before clean reinstalling.

As long as files are backed up he has nothing to lose.

First the good news. Many thanks for the patient and helpful advice from Alphanumeric and Gregrocker in telling me that there is a way to do what I desired... and further thanks to Gregrocker for pointing out where I deviated from the trail that was shown in his screen shot but I was too impatient to follow to the finish. The registry has been successfully modified.

Now for the bad news. The repair install to do the upgrade to Home Premium from Professional freezes at a point when the end is in sight. It is so close I can almost taste it and feel it in my hand. The freeze occurs during the Transferring Files, Settings, and Programs portion. The display is at 72% of completion. But that figure is misleading. It is actually at 99.9729+%! There are only 77 items left to transfer. This first pass at the install was with all the recommended settings for the install in place including the internet. Then I tried a second time. Everything the same except no internet for this attempt. Except for the total number of items to be transfered being a few less the result was the same. Another freeze with only 77 items to go.

Now I am off to do more studying and research on the forum for ideas for the cure to this problem.

Hopefully it rolled back. The last entry in any of the following logs should give you a clue as to what is hanging:
C:\$WINDOWS.~BT\Sources\Panther\setuperr.log
C:\$WINDOWS.~BT\Sources\Panther\setupact.log
C:\$WINDOWS.~BT\Sources\Panther\miglog.xml
C:\$WINDOWS.~BT\Sources\Panther\setupapi\setupapi.dev.log
C:\$WINDOWS.~BT\Sources\Panther\setupapi\setupapi.app.log
C:\$WINDOWS.~BT\Sources\Panther\PreGatherPnPList.log
C:\$WINDOWS.~BT\Sources\Panther\PostGatherPnPList.log

None of this is a good omen for a perfect in-place Upgrade. Best to wipe the HD and start fresh.

Are you ready to clean reinstall? re-install windows 7

The saga continues in a new thread Repair Install Freezes at 72%