I think I found it. I go to the Nvidia Control Panel. Under the "Display" area, there is an option called "Adjust desktop size and position".
Here are the options it gives:
Scaling / Size
When it's under the Scaling part of it, it shows:
Select a scaling mode:
Aspect Ratio (X)
Full Screen ( )
No Scaling ( )
(Aspect Ratio is the one currently selected)
Perform scaling on:
(there is a drop box with these options:)
It's currently set to Display, so I'm guessing I change it to GPU.
There is also a box that says:
Override the scaling mode set by games and programs ( )
(that box is currently unchecked. Should I activate it ? )
Then it has a preview area.
It has a resolution dropbox
a refresh rate dropbox
and then at the bottom, it shows the current native resolution.
(right now, I have my PC hooked up to my regular PC monitor, so it's showing it at 1920 x 1080 and the refresh rate at 60Hz. I don't have it hooked up to the projector right now, but I'm guessing that box would show 1280 x 720 and 60 Hz, with the Native resolution at 1280 x 720)
Later this afternoon, I'll hook the PC back to the projector and test some stuff out. I'll go ahead and set "Perform scaling on" to GPU, and I'll also mark the "override the scaling mode set by games and programs" option. That might screw things up, but if it does, I'll just go back and uncheck it. I have the original Portal, so I guess I can try it with that.
Oh, by the way, when I'm changing custom resolutions, for some reason, it seems to want to use 61Hz instead of 60Hz. What I mean is, when I first change the resolution, it automatically changes the Hz to 61. I change it back to 60, and then it seems to save it, but just wondering if maybe I should be leaving it at 61Hz for some reason ?