Help with a virus

Hello,

 

I have a computer that is not currently running Norton Internet Security.  It is infected with something, but I don't know what it is.  I'm looking for information on how I might be able to remove what ever it is and recover the system without having to re-image the hdd.

 

When I log in to the system, a message displays saying I'm not running genuine Microsoft software.  It then offers an option to get genuine Microsoft software.  I cancel out of this dialog box, but then the wall paper just goes totally black and in the lower right hand corner the same message is displayed - Not genuine Microsoft software.  At this point, my network connection is disabled and I really can't do much of anything.

 

Any ideas what this is and how I can remove it?  Is there a tool I can download onto a ram disk and run from a USB port to clean this?