Multiple Monitors?

I run Win7 on my laptop. I use it with a seperate external large
monitor. I really never use the laptop monitor, except when I move
the laptop.

Wjhen I go to the SCreen Resolution place in Control Panel, my
computer shows three monitoprs:
The external
the lap top
a third which sems to come from no where.

Fot the third monitor, I select the pull down option that says
"Disconnect This Monitor", and I guess it's disconnected. How do I
remove it? Get it off the options?

Thanks!