If you set the resolution to 800x600 on the existing display, then plug the
touchscreen into that video connector, does that work ?
When you get the code 10 in Device Manager - where does it show up ?
To find examples of what the control panels might look like, it would help
to know which model of NEC powermate this is. There are a ton of different
model numbers. And using utilities to list the hardware in the box, is better
than someone here trying to figure it out with web searches.
There is an article here, showing a couple of tricks. The Display control
panel would show two monitors, if both had been detected, and both hardware
and software supported two displays. In the picture here, one of the displays
is greyed out.
These utilities will give you some info about the hardware inside the computer.
CPUZ will help identify the motherboard and motherboard chipset. The other
two may be able to help with cards that are plugged in, such as what kind
of video card this is. The Display Manager in Windows can also give
some info. (Everest - Devices:PCI will show a lot of hardware - it includes
my AGP video card.)
http://www.sisoftware.net/dload/sware_figure.php?&a=&langx=en (Sandra Lite)
To give an example, I can look in my Device Manager. I see
Radeon 9800 Pro
Radeon 9800 Pro - Secondary
Those correspond to the ability to drive two monitor connectors off the
same faceplate on the video card. If I do "Properties" on those two, the
same drivers are used for both of them.
Some video devices have nice manuals, and some don't. But knowing a bit
more about your machine, may make it easier to dig up stuff to help.