Over the past couple of years for no apparent reason occasionally my screen resolution would change upon startup.
My correct resolution 1440x990 would not evevn be in the list, being changed to 1280x960.
I have always been able to correct this problem by doing a restore to an earlier date.
This week the same thing happened but this time a restore did not work.
I am now stuck with my screen resolution at 1280x960, being the nearest I can get to the correct resolution.
I am using Windows Vista Home Premium, graphics ATI Radeon HD 4350.
Unsure but I think, in your shoes, I'd make a start by checking you have the latest drivers for your monitor. And it sounds as if your graphics card may not be up to giving the resolution you want for your screen.
Also check the device manager to see if there are any exclamation marks or crosses against your video devices. Could give a clue.