I dont know what the problem is sorry, but i have an alternative for you (so that you can unzip/open the files until this is fixed)...
You can use 7Zip as an alternative Zip/Unzip program... it's here:
http://www.7-zip.org/
As for the actual problem, it could be malware based, it could be missing files/damaged files... Anyway i suggest you read this thread and follow the instructions:
http://www.techsupportforum.com/secu...oval-help.html
(Incase it is malware)
Cheers,
Jamey