With all the virus warnings I regularly get in email (most of which are hoaxes), I never heard of the email I received today which was most obviously a virus or worm carrier. Here's what it looks like:
As for what the attachment does, there seems to be a difference of opinion in the "blog-o-sphere".
One blogger said it was something called "malvertising" software which was downloaded when the unsuspecting recipient opens the email. He said he almost fell for it. It, according to him, installs fake "spyware" which tells you that your computer is full of viruses and then, tries to sell you a solution.
Another blogger identifies it as a worm/trojan carrier but he doesn't specify what mischief it might do.
The Microsoft website identifies the payload as a worm but again doesn't specify much about what it does.
Whatever the case, there is a really good chance you do not want it in your computer so if you get an email looking like the one above, don't open the attachment! :)