ther are many other computers in my house where fullscreen works perfectaly but i would rather use my own computer with it

I suck at this screen/size stuff, so all I can say is check the 'output=' settings in the dosbox.conf file. Try changing 'output=' to one of surface, overlay, opengl, openglnb, or ddraw.

Also, try setting 'fullfixed=true' (perhaps you also need to change 'fullresolution=' to something your monitor like).

Also try setting the fullresolution= to something you know your monitor/video card supports.

Thanks the help, I justed changed the output=surface to output=openglnb and i changed the fullresolution to 1280 by 1024 and it works like a charm, no i can play all my class games in fullscreen and it looks excellent.

just a small question, whats the difference between opengl and openglnb?

I don't have an official answer, but I believe the difference is that openglnb stands for "OpenGL, no bilinear filtering", which means that it won't use OpenGL to smooth the graphics (which output=opengl most likely will). The OpenGL modes are actually a bit slow for me compared to the other modes for some reason, so I prefer ddraw or overlay myself (of course, your mileage may vary)

Yes. Many people say OpenGL rendering mode is more suitable for nVIDIA cards while ddraw or overlay would be better for ATi cards. As HunterZ has stated, your mileage may vary. You have to try several values until you find the one you feel more comfortable with. 😎

PS - The difference between OpenGL and OpenGLnb is what HunterZ said. OpenGL enables bilinear filtering on textures by default (tends to make things a little blurry in my humble opinion. You will have to try it. Some people uses it for smoothing out textures. I don't like it personally). OpenGLnb is exactly the same OpenGL rendering mode but without bilinear filtering.