The error message refers to a .pvd file, not a .bak file as you were originally asking about. Which is it? Whichever, eliminate the external drive as the source of the problem by copying the file to a local drive. When you start PVD, you'll get error messages because the program is going to try to open the last used file. Dismiss any messages and proceed to opening a file...
If the file is a .pvd file, you should be able to Open it. If necessary, the program will update it to the current format. If you have a copy of the original .pvd file, use it—that's less complicated and error prone than...
If it's a .bak file, you won't be able to open it directly. A backup file is meant for restoring a current database. I believe the program has to have something loaded, or it won't be able to do a restore. So Create a new database, and then restore.
I suppose the problem might also be UAC. What version of Windows are you using?
