I'm fiddling around with an old Pentium-M laptop, with GMA 900 graphics.
The only downside I found so far is that for 640x480 games, they always stretch to fill the whole screen (16:10).
Setting the desktop resolution to 640x480, 800x600 or 1024x800 also results in a stretched image.

Is there any registry value, driver hack or config to force it to maintain the correct aspect ratio?

The Intel Graphics software has an option to "Center Desktop", which results in a non-scaled display, so for example, 640x480 keeps the right proportions but it's quite small and centered.

There must exist some solution because on GNU/Linux this limitation is not present.

Otherwise, I'm surprised by how well the integrated graphics perform.