What do I do with this "spam" mail?

Okay I got this message. In the username it said "notice from the FBI." and the message subject said "anti-terrorist and monetary crimes division!!!" yes, with the exclamation points. It has a doent attached, and under it it says "from the FBI, see attached letter for details" and then the signature says "FBI Director, Robert S. Mueller, III" I don't want to open the letter, because I'm 95% sure it's a virus, but I'm just really really curious. so has anyone else been stupid anough to open one of these letters? if so, what was in it?