How can i stop steam from changing my display from 1024 by 768 to 800 by 600?

everytime i turn on steam, it changes my current display screen resolution which is 1024 X 768 to 800 X 600 and from that point i have to change the display manualy, any way to stop this?How can i stop steam from changing my display from 1024 by 768 to 800 by 600?
Try lowering game quality settings in the game's graphics option. If game quality setting is set high, Steam could be detecting that it is too high for your graphics card to run smoothly at 1024x768. Steam is trying to compensate by lowering display resolution.