The actual culprit is TMM, Vista's process that looks for multiple monitors and attempts to intelligently modify resolution. If you aren't using multiple monitors, you can disable TMM:
1. right-click on 'computer' and select 'manage'
2. on the left pane, expand 'task scheduler' then ' task scheduler library' then 'Microsoft' then 'Windows' then 'MobilePC'
3. select TMM in the center pane, then on the right pane, choose 'disable'
I disabled the TMM on my Dell Inspiron 1420, running Vista Home, because it was doing the same thing... upon logging on, either from cold start-up or from sleeping/hibernating, the screen resolution would reduce greatly. Disabling the TMM did not help me. The screen resolution reduces still. Any other ideas?