What card do you have?
If its an Nvidia, go to Control Panel, and then to the Displays controls. In there, probably under the advanced tab, you shoud find the Nvidia Control panel. In there will be a wizzrd or other way to access the 3D controls.
It depends on how you have cnfigured Nvidia and which version of the Nvidia controls you have.
If its ATI, then I don't know where it's located, but it should be a similar route.
Make sure that you have the latest video drivers installed, and try googling
"3d control panel nvdidia spanning mode". (or ATI instead of Nvidia).