Xbox 360 on A PC 21" Flat Screen?

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Searching is always a good thing, but hostility toward the original poster can be done without, in my opinion. Why don't we focus on effectively answering the OP's question instead?

the baker at jury lane:

First, buy a VGA cable for your Xbox 360. You can either buy the Offical Microsoft VGA cable, or you could go with this Psyclone VGA cable for a cheaper price but equal quality alternative.

Second, based on your setup and as suggested by halifax1, you will need a 3.5mm to RCA adapter (otherwise known as a 3.5mm Stereo Splitter). You'll want one that looks like this.

Lastly, you'll plug the red and white (RCA) plugs from the Xbox 360 VGA cable into the 3.5mm Stereo Splitter and can probably just leave it on there. When you want to play your Xbox 360, unplug your computer's speakers and plug it into that adapter (which should be attached to your Xbox 360 VGA cable now). Then unplug your computer's VGA cable from the back of your monitor, and plug your Xbox 360's VGA cable in. All that's left is to power on your 360 and start gaming.

Good luck, and have fun.

The VGA cable comes with a rca to headphone splitter already, so all you will need to buy is the xbox VGA cable and you are all set
edit: you might want to get a female one like the post above^ if you have to connect the xbox directly to external speakers. You could plug the included one into the mic/line in port on the pc though and use that for audio(through the pc)