57% of all VM launches in Virtual PC are failures. I’m joking, don’t believe everything you read on the internet (or politicians for that matter). To that end, take my solution into consideration but a warning for you, there is a distinct chance you will lose data in this case, especially if you never save changes from the Undo Disk.
If you are having problems booting your VM in Virtual PC, I have a trick fro those of you who have the Undo Disk enabled. If you cannot boot, you can go into settings of the VM > Close in the left pane > Select Prompt in the right pane.
Try to stop the VM and it will prompt you for an action when you close. Select “Turn Off and Delete Changes”. When it closes it deletes everything in your undo disk and it will revert your VM to a time when, hopefully it was booting correctly.