Hi,
Below steps may applicable if it is Password protected and not password encrypted as you mentioned.
1. Download the attachment file and store it in a local folder. (Get Mail or Read Mail will do this)
2. Un zip the file using any Unzip object if you have. If you do not have Unzip object, please try to create one
3. Make sure that Unzip object accepts password to unlock the file and Unzip (if you do not have any object, you can create one by searching in the net)
Hope this helps.