Aspect ratio problem

Hi. I''m starting with D3D and I''ve made an app that shows a textured square rotating around an edge. I''m running it in fullscreen, and the problem is that my square becomes a rectangle when rendered. It deformates, showing wider than it really is. I wonder if that''s because of the Aspect Ratio value in the D3DXMatrixPerspectiveFovLH call. I''ve tried with my screen width/screen height (800/600) and with 1.0f, but I don''t see any change. So, do anyone know how to make my square look like a square? Thank you.