So long as your graphics card allows you to select a mode where the resolution matches the resolution of the monitor you wont have any problems with the aspect of circles.
For example if your widescreen's pixel resolution is 1600 x 900 (16:9 aspect ratio), and you choose the same resolution with your graphics card then you will have square pixels, and circles will be circles.
But if you were to leave the resolution of your graphics card at, say 1024 x 768 (4:3 aspect) on your widescreen monitor, your circles will be ovals.
Almost all screens are manufactured with square pixels - all you need to do is match the graphics card resolution to the screen resolution and all is fine.
I run use a wide-screen monitor myself and my circles are as circular as a very round thing
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"