problems with bitmaps

Whenever I open a bitmap in Photoshop, the visual quality of the image is not as good is it is in ImageReady. The bitmap looks exactly right in everything but photoshop. Even ms-paint! The problem doesn't happen to Jpegs though. The effect is something like a stepping-down effect between shades(or colors?) and it ruins the entire image. (eg. a bitmap image of an apple looks like it has obvious lines of shading)
I've picked photoshop apart and I can't figure it out.
What should I do?

are you in rgb or aomething like index , etc? and check out your color settings

I played around with those color settings just like I played around with an array of other settings and they did not help me. I still don't know exactly what it would be. I may have missed something, like a combination of settings that would fix the problem. But, I need to know more.

did you try switching the mode to bitman when you start a new image or any of the other modes?

I tried that too but they don't affect the bitmaps I open. Only the new ones I create.

check your colour depth settings under windows (under settings>color pallete)