I want to share both Windows 8 and Windows xp screen with one single AOC monitor, for more multitasking.
The obvious answer is to use a KVM (Keyboard, Video, Mouse) switch, which are available at a wide range of prices. However, if your screen has multiple inputs (e.g. VGA, DVI, HDMI) you can simply connect to both computers using different inputs. It's then just a matter of pressing the "source" button on the screen to switch between the computes.
The easiest method would be to use a KVM switch but it depend on the monitor input and the keyboard and mouse used. Most modern computers use USB ports for keyboard and mouse. Many KVM switches are aimed at the older 15 pin VGA monitor ports. Luckily most modern monitors have multiple ports so your choice of KVM switch will be more open. an example of the switches are listed below; notice NewEgg allows you to easily select the options you need for your setup.
If you prefer to use one computer to control another through remote control software like VNC that would be another option. A quick article explaining the basics of VNC is at http://gizmodo.com/5197759/how-to-remote-control-your-home-computer-from-anywhere-with-vnc Good luck with your setup.
If your monitor has any two of DVi, HTML (both digital) and VGA (analog) port just connect both computers to the monitor. An example would be to connect laptop using VGA or HTML and desktop with DVI. Monitor has Source button you can click to select computer you want to use.
Depending on what you are doing, it may be easier to simply remote control one of the computes with TeamViewer.
How to Use One Monitor with a Laptop and a Desktop