Another guess, but possible....
I do not keep up with the latest hardware, but I have heard that the XP desktop will not go below 800x600. I have not needed to install a resolution switcher on my XP laptop so I don't know if that is true. But if it is, like
Inkybro has suggested maybe your motherboard has a new-fangled graphics card built in that does not support a 640x480 desktop? That probably makes no sense since it should have to run 640x480 apps, but who knows nowadays?
The more you see, the more you know.
The more you know, the more you see.