Here you go:
http://moschini.org/945GM-800x480.zip
This is a custom-built driver package for Windows XP, using the IEGD 6.1 configuration editor.
It is configured with one custom EDID for 800 x 480 on the Analog VGA port, and should use 800 x 480 x 32 bits by default.
Extract the package, go into the Utilities folder, and use the Setup.exe program to install the drivers.
Oh, and it couldn't hurt to uninstall any other video driver packages you see in the Add/Remove Programs control panel before you do this..
Let me know how it works!