On this site:
www.s-video.com there are some demonstration videos showing different setups maybe Worth a visit.
It varies a bit as to whether your using XP or Vista and what video card you have. So this instruction is for Vista. with Gforce.
Right click a blank area of your desktop. From the fly out box select Personalize, then display settings. Highlight the second monitor, tick the box extend my desktop to this monitor. Then select advanced box, you should see a tab on the top showing your graphics card open this. you should see another box Control panel, on opening this you should see buttons to select type of dual screen .Pick dual view not clone.
You must have the second monitor connected and switched on to do this.