Dammit. It happens from time to time, I have no idea why. It tends to happen when the file is a little older, but then just fixes itself.
Anyway, 8.4 is the most recent version: http://www.mpgh.net/forum/showthread.php?t=783708
8.4 is having the same problem.
But it's not a problem unique to you (I can't unzip it right now either). I've also tried extracting it with a different application I have and it worked, but I can't actually open the working file within I'll update you if I find anything.