I don't know if you've got ATI or nVidia control center installed, but that's the software you'll need. I'm fairly sure there will be an option to extend your desktop somewhere in that software. Note, you'll need a reasonably powerful GPU to run dual displays well at high resolutions.
Ok, so i can link my laptop up with my tv, and i do this quite often to watch films! But is there a way i can use my tv as a second screen? So i can watch a film from my laptop on my tv screen but still use laptop at the same time? Im sure i have seen someone do it before..!