Looks like that is not exactly a...legit copy

Most websites like that set a password on their archives so that other places find it harder to leech off of their DL. If you can remmeber where you got it from, then try that site again, check if their is a PW given.
If not, try WinZIP. Maybe your ZIP program just isn't interperting the files corectly. It might just be a fluke that you can't open them now, but could before. They might be new copies, or accedently set when you didn't realise it.
Armani