Unzipping zip files
 

Hi there,
please can anyone help me? Windows will not let me open zip files. It comes up with various excuses most of the time saying that the file is corrupt or there is a virus. I know that this can happen but they can't all be wrong un's can they? Even zip files from friends that I know are safe don't open. I have Winzip and Power Archiver but still they remain firmly closed.
Anyone got a sledgehammer? :D