Resolution changes after rebooting

I have a Laptop Compaq Precision 565LA, with Windows Vista. I use resolution 1024x768, but recently after a reboot it started changing. Actually when I reboot or start up from powered off, the computer acts normal. But in the process of loading up all the desktop icons and system tray, like 45 secs after the desktop is on the screen, it flickers and changes automatically the resolution to 800x600. I go it to settings and change the resolution back to 1024x768, and it works perfectly with it during the time the system is on. But after shutting down the system and starting it up again, it does the same thing.

I have done virus and spyware check, but it didn't find anything. If someone could help I would highly appreciate it.