since I have an HP slimline and couldn't find a video card that would fit (the installed video card does not support dual monitors), I purchases a Tritton USB VGA video card to run dual monitors. The monitor works fine but I can't get "extend these displays" to work in Windows 7. Control Panel display utility allows me to select dual monitors and select the "extend" option but I get a "duplicate" on the second monitor (sort of duplicate ...not actually in real time). I'm an editor and want to be able to view multiple documents at the same time (resolution isn't a big issue, even though the resolution is low for an Acer HD monitor).
What is the native resolution of the monitor that is connected to the TRITTON USB device?
Are the current Windows 7 device driver for the TRITTON USB device installed?
Are you running Windows 7 in Aero Mode? If you are, then disable Aero Mode, and try extending to the second monitor.
There are several low-profile video cards on the market that would be more suitable for your purpose. The only limiting factor for the HP slimline case may be the power supply rating. Since you haven't mentioned the model of the HP computer it would be difficult to guess its power rating or suggesting available low-profile video cards.