Norton Internet Security Scan Software

I left my computer idle for about 90 minutes yesterday and when I came back, I found that "Norton Internet Security Scan" had been installed.  There was even a shortcut added to my desktop.  Once I found out it was legitimate, I ran it and was immediately told that I didn't need it since I already have NIS 2009 installed.

 

I've now uninstalled the software, but I have no idea how it got installed in the first place.  I didn't request it.  I hadn't even heard of it until yesterday.  Did this happen to anyone else??  Does anyone know why it happened??