I have a Dell Monitor and I'm working with resolution 1152*864 although the recommended resolution is 1024*768, I noticed lately curved lines at the right and left edges of the screen looks like the lines of a magnetic field, these lines appear only at 1152*864 resolution and disappear on higher or lower resolutions, I changed the refresh rate but nothing happened...so any ideas?

BTW these lines appear only on specific background colors like gray and green backgrounds but nothing appear on a black background for example...
Also I don't know if this has to do anything with the problem but I installed windows 7 lately and I noticed the problem after that, so I don't know if this the problem or its just a coincident...

...the recommended resolution is 1024*768

This implies an LCD type monitor. Using non-native resolutions results in the image being scaled to fit the native resolution. Some monitors scale better than others.

...looks like the lines of a magnetic field...

This implies a CRT type monitor. LCD monitors don't have a magnetic field.

...these lines appear only on specific background colors like gray and green backgrounds...

This sounds like moiré (http://en.wikipedia.org/wiki/Moir%C3%A9_pattern) or an artifact caused by a LCD monitor with a slow response time.