Google filters viruses sent or received through Gmail by using a heavy handed approach to filtering. Rather than scanning each email for viruses, they simply prevent people from sending or receiving files with .exe extensions, even if they are in a zipped folder.
Getting around this is very simple, but you must let the recipient know what's going on.
Anytime you want to send a .zip file rename the extension. I like to just take the "p" off of the end. For example, I will name the file - file.zi. In the body of the message I tell the recipient to add the "p" back on before trying to open the file.
Links:
[1] https://www.paypal.com/cgi-bin/webscr?cmd=_xclick&business=rick@yourpcpartner.com&item_name=Buy
[2] http://www.helpmerick.com/node/1364
[3] http://www.helpmerick.com/node/1361