Here's a trick - I was capturing A/V from the Camcorder and tried to cancel the recording - MVPX3 crashed big time - and then crashed Windows XP SP3. The Blue Screen of Death came up and said something about dumping memory to a file.
Sometime later, Windows finally rebooted.
Anyway, MVPX3 would not run after that. As luck would have it, it was late and Tech support was not available at Serif. (They have holidays and time off too - as well they should!)
I tried to re-install it, and it would not install. I then used the new Windows Install Clean Up utility to remove MVPX3 so a fresh copy could be installed, except, MVPX3 would not complete the install. Several attempts later, I finally went to the Crogram FilesSerifMoviePlusX3Program folder and manually deleted everything in it, folders and all. After re-booting the PC, MVPX3 installed just fine. The latest patch was also installed and everything is running fine.