Hi Eric
i have seen this also when installing it the first time a long time ago, but i can't remember what the solution was.
A few suggestions:
Make sure you point the path to the correct .exe file
Make sure that your windows user account was writing access to the folder where AnotherGUI is located. Normally "Program Files" folder needs administrator access to be able to write files without the poup dialogbox. I have my AnotherGUI installation just in a folder on my desktop, not the most elegant solution, but it works for me. :-)
Now i only get that error when i by accident try to overwrite the same file as i am converting, remember to rename the output file, or set another folder as destination.
Hope any of this can help.