Teamviewer remote computer 640x480 screen?

Try this :

1. Access Control Panel -> Appearance and Personalization -> Display -> Screen Resolution -> Advanced button.
2. In the Adapter tab click List All Modes.
3. If not enough modes are shown, try to uncheck the Hide modes that this monitor cannot display checkbox in the Monitor tab.
4. Click on a resolution to choose it
5. Click OK twice.

If installing the latest graphics driver hasn't helped you, then this might.

It's a tutorial on how to create a dummy VGA plug so your computer thinks you have a monitor connected. However, I haven't tried this out myself so I have no way of knowing it'll work for sure.

I found out about this from a Super User answer here. Let me know if this helps.

@ChrisN comment is the right answer. I'll post it as an answer to the readers:

In my nVidia Control Center, I can go to Display-->Change resolution, click on Customize, and click Create Custom Resolution:

After that, select the checkbox that adds it to the list of resolutions and select in on the main "Change resolution" screen.

There is no Screen, so Windows Detects that there is no screen.

all you have to do is connect a monitor to it, you don't have to use it, just connect it.

another option is to purchase a Dummy Dongle, it sounds like there are a few of them out there if you know where to look, another Superuser was able to find this DVI-Dummy it seems pretty expensive to me, but I have never had the need to do something like this before.

another option is to look for people that do stuff like THIS I don't really know what they are talking about with the EDID Dongle, but it sounds like you can make the graphics card think there is something of a certain resolution connected to it.