What video card do you have? Monitor size/native resolution? Did you install the video drivers?
Sometimes windows does not recognize the monitor and uses the default plug n play monitor. Make sure it says the monitor brand and model correctly. If not see the monitor manufacturer's website for monitor drivers.
I have the Amd radeon hd 4670 series and the monitor worked fine with my old graphics card a zotac geforce 210 I have installed all the catalyst drivers and stuff. My monitors native res is 1920x1080 and it is the Acer slimline 24" LED monitor.