You cannot get the slider to go as low as 640 x 480 and you cannot get the color depth below 16-bit?
Microsoft no longer supports 640 x 480 resolution or a color depth lower than 16-bit.
You can try the following unsupported workaround:
1. Open the Display Properties dialog and select the Settings tab.
2. Press the Advanced button.
3. Select the Adapter tab.
4. Press the List all modes button.
5. Select the resolution, color depth, and refresh rate.
6. Press OK.