Virtual PC, XP mode, sudden fatal error 108

Starting today, when I try to start my XP mode I get a combination of a warning and a fatal error as partially shown below:
...
Event ID: 87
Task Category: None
Level: Warning
'Windows XP Mode' was turned off without hibernating state.
...
and also
Description:
'Windows XP Mode' experienced a fatal processor error and has been turned off.
Event ID: 108
...
I get this whether I attempt to start it from the desk top Icon I had created or from the file
Windows XP Mode.vmcx (which says "powered down").

I have no idea why this might have happened. I've done web searches and see similar comments. Some of those comments say to re-install virtual pc from the microsoft web page. When I try to do that, what gets downloaded is a file called:
Windows6.1-KB958559-x64-RefreshPkg.msu
and when I attempt to execute that I get a message that says:
The update is not applicable to your computer.

I got no replies to this problem in this forum, but have managed to fix it based on a few postings (out of many). The final solution after many failed attempts was to 1. Turn off hardware virtualization in my Avast and 2. do a clean boot (i.e. disable all non microsoft services from loading using msconfig). After doing that my xp mode opened as it should. It is still working after booting with all services and hardware virtualization turned back on.