When i turn on my xbox 360 a red light comes on the computer

oh no. youve got the red ring dude. no fixing it. get a new one. your screwed with that thing. im sorry

on the ring of light there are 4 lights
1 light = hardware malfunction
2 lights = overheating
3 light = RROD (red ring of death) your GPU has come loose
4 lights = a/v cable is loose/bad

Take out the dvd drive and then turn on the power to see if the one red light comes on. If it doesn't come on after taking out the dvd drive then that means the dvd drive is shorting out the main board.

If you take out the dvd drive and the one red light still comes on, then that means its a problem with the main board and your 360 would need to be serviced.

also know as the ring of death, its a software issue and you need to replace your system

When you get any of these messages appearing on your screen with one red light flashing on the ROL of your Xbox 360 console that's an indicator of a hardware problem with the console connection to one of the parts connected to it as the hard drive or cables.
So, first disconnect all power and AV cables from the back of your Xbox 360 console and also unplug the power cord from the wall socket, wait for a minute then reconnect them all back and try to turn on your Xbox 360 console once more.
If that doesn't work, it seems then to be a fault with your Xbox 360 hard drive so turn off the console again and remove the hard drive then turn on your Xbox 360 console without the hard drive, supposed then to work fine, it might be an intermittent issue and when you reconnect the hard drive your Xbox 360 will work fine all after, or it will just work fine without the hard drive so it will be then definitely a fault with your Xbox 360 hard drive so call Xbox 360 customer support to exchange the hard drive for you.
The number for Microsoft is usually listed on the back of any game manual.

I hope I was able to fixya problem, and Happy Gaming!

This problem may occur when the Xbox console is too hot or is overheating. The flashing lights mean that the internal temperature of your Xbox 360 console has exceeded acceptable temperature limits. Let your Xbox 360 console cool. You may have to wait several hours for your Xbox 360 console to cool enough. Don't turn on your Xbox 360 console when the console is hot.Verify that your Xbox 360 console has enough ventilation and that the fans are operating. If the two lights continue to flash red after you try steps 1 and 2, contact Xbox Customer Support.

Microsoft can be contacted by finding the number listed on the back of your video game booklets.

Good Luck to you and Happy Gaming.
Source:
http://support.microsoft.com/kb/907533

check this website and follow the steps in it:

http://ezinearticles.com/?Xbox-360-3-Red-Lights-Fix---The-Simple-Solution-To-Your-Xbox-360-3-Red-Lights-Fix&id=1201271

http://www.360-hq.com/xbox-tutorials-23.html

Go here to answer all your question. I promise you.

Billy D

This means your 360 has had a hardware failure. If you have purchased your XBOX 360 within the last 3 years you will still have free warranty on it. Call XBOX customer support and they will help you with replacing/repairing your Xbox 360. 1-800-4MY-XBOX. (1-800-469-9269)

Hope this helps :)

Make sure that the AV cable is correctly connected to the Xbox 360 console.

Disconnect the AV cable from the Xbox 360 console, and then reconnect the AV cable to the Xbox 360 console.

If the four flashing red lights continue to flash, try wiping the metal area of the AV pack with a dry cloth. The metal area is the end that plugs into the console. Wipe the metal area thoroughly, and then try the AV Pack again.

If the AV cable is correctly connected but the four red lights are still flashing, substitute a different AV cable if you have one available.