Use DVI-D (Dual link). VGA has a maximum resolution cap due to bandwidth issues; DVI is an XP era cable that would be able to carry full 1920x1080. If you have HDMI or DP, those will do the job too. Update your graphics drivers too, it'll help.
I updated graphics driver, but it didn't help. I don't have HDMI or DP...I have ordered a Graphics Card from Dell. According to Dell this will fix my problem. Thanks for you reply.