Normally, this can only mean the download is corrupted. I don't expect that the program you use to unzip it or your antivirus makes a difference. But it could be a problem in your browser, your download manager, the connection or the ISP.
The first thing to try: download on another PC and put on a USB-stick. Check if it can be unzipped there (you don't need any special program to do it, both XP and Vista do it natively).
Compare the file size. If different, unzip the new one on your machine. What happens?
ive downloaded a few files from the net from authentic sites like softpedia.com but when i unzip them i get an error "unexpected end of archive",
1. what does this message mean
2. is it happening cos i i have a vista and there may be some compatibility issues
3. is my antivirus(kaspersky) responsible for this, it might be detecting the file as a threat?