Since my GPU is pretty old,i like to play at lower resolutions.I have just bought an HP 2009m 1600x900 monitor,
and i can't change screen refresh rate on my games like my old monitor so now i have black bars on the side or top.
I have the latest drivers from Nvidia and my connection is DVI-VGA adapter.
k1114 is right. That has nothing to do with your refresh rate... You can get rid of the black bars through the nvidia control panel by disabling scaling, but your game will look blurrier (because it won't match the resolution of the screen) but it will fill up your screen. Your options are basically, black bars with sharper details, no black bars, but blurrier, or change the resolution to 1600x900 and have lower framerates but better looking all around.