The error occurs because the GUI is trying to process information that wasn't generated when the Open File button is pressed. The #1 cause is that the GUI can't see the Mobi2Mobi.exe. If you've set the directory and the Mobi2Mobi.exe is named exactly as here then its something new. Bummer. It will be interesting to see if you have the same problem under WinXP. I'll try and get around to building a Vista VM Appliance and do some testing.