Well, I manage to solve the problem of no longer being able to double-left-click within LW and launch automatically in Windows Media Player. I just opened WMP separately, and chose Tools->Options->File Types. "MP3 audio file" and "MPEG movie file" were already checked, but unchecked and rechecked them, clicked Apply and then OK. That seems to have done the trick!

Also, I must thank Lord of the Rings again for suggesting VLC. It's marvellous for previewing AVIs! No more wasted days downloading corruped movie files!