Problem with ArcadeVGA and autopan mode

[Serpent]

I'm using my ArcadeVGA at 648x288 (non interlaced) with autopanning mode (with a virtual desktop of 1024x768). This is under win2000. Everything works perfect, except for one thing. When I use MAME32 games with low res non-interlaced modes there's no poblem. They look perfect in their native low res modes. But with games using higher res mode they look as if they also were in a virtual desktop, so I can only see the top left corner. I presume that when, for instance, 640x480 is selected as res mode then windows understands that I want to use a virtual screen, so the actual res keeps on being a low res non-interlaced. Can this be fixed with MAME's configuration? I wonder if these games would look fine, as before, if I disable the virtual desktop. I've tried however, and I cannot find out how to leave my windows config as it was before I set it to use pan mode. It's pretty confusing. I'd really like to know if there's a way to switch between enable and disable. I really love this autopanning but I will not use it if it affects games which require high res modes.