I guess one of the OS's .ini file has been corrupted and that's what made the default explorer.exe association to the drive to be null.

You can try...

1. When the popup opens try associating it with explorer.exe(i think it resides in c:\program files\explorer.exe and make sure you check the option always open with.

2. Can be also be fixed by replacing the .ini file with the appropriate settings or by reinstalling your windows.