I use dual monitors as well, but as a clone. How does yours work? Right click the desktop and click properties. Go over to the settings tab, and click your monitor, the 20 inch. Try changing then. I know you somehow turn off the Laptops monitor, I have a Dell Laptop and you press Ctrl Alt F3 to switch the onboard monitor on and off.
I have a 20.1" DELL monitor that I use with my notebook in my office when I'm not traveling. My setup used to be that the monitor on my notebook had a different resolution (1024 x 768) than the external Dell monitor (1680 x 1050). Something happened and now I don't have that 1680 x 1050 resolution as an option, and the external monitor and laptop monitor are now set the same. I redownloaded the drivers from the Dell CD that came with the monitor, but I still can't set the resolution correctly.