Festive,
I wouldnt recommend opening it, your computer may have a worm in it, especially if it prompts you for a password, that sometimes means you may have downloaded a program, that has a trojan in it, that captures your keystrokes, so they can have access to your passwords on email, and get into your files. Run your anti-virus asap. You have kids that download music sharing files, or game files? You may have gotten it this way. Im not saying for sure you have one, but that is the usual indication of trying to open an attachment.
Trish