1) make sure the file was downloaded properly. this happenes on all unzip programs, if it is incomplete it may think there is a password needed or may not work with it at all.This is what it says for me.
2) download winrar, 7zip programs, they are free for a time (though usually people will deal with the pop up ads after that time is expired but you really should support the program. even windows has it's own version of pczip built in you could use.
I'd say choose the lesser of two evils, download a different program first will be probably just about as fast as re-downloading the main file.