By the way you said that, I'm assuming you use Outlook Express
If you use Outlook Express, you click on the email (single click) and it opens in the preview window. So yes, it opens it, but USUALLY* you have to open an attachment for anything bad to happen. There is a way to make it so that there is no preview window, so that to open an email, you have to double click it. It should be in one of the preferance things (nice accurate description, huh?)
*There are some virii that all you have to do is open the email, and not the attachment. Download the newest patches for Outlook Express. It doesn't make it bullet-proof, but it will definately help.
If you use the full version of Outlook... probably the same thing, but I don't know offhand.
If you use something else, I can't help ya there