One Light
One flashing red light is displayed on your XBox 360 when a hardware failure occurs on your system. If you turn on your TV, you may see a note about a system error and directions to contact Microsoft customer support. Xbox Support suggests that you disconnect all cables from your system and plug them in again if you receive this error message. Do the same with your hard drive or any memory cards. If the problem persists, then contact XBox customer support.
Two Lights
Support.Xbox.com, the Microsoft XBox official support site, notes that two flashing red lights indicate that your XBox 360 is overheating. The easiest solution is to turn the system off and let it cool down. Making sure the unit is in a well-ventilated area free of clutter where it can "breath" will help the system stay cool. Third-Party companies also sell exterior fans for the XBox 360, as it has a reputation for overheating somewhat regularly.
Three Lights
Three flashing red lights indicate a hardware problem with the power supply or console or both. Turn off your console and unplug the power cord completely then plug it back in. If the problem persists, try using a different wall socket. If the power supply is overheating, the brick light will turn red and should be disconnected long enough to fully cool. If the problem persists, you may need to replace your power supply.
Four Lights
Four flashing red lights means that your XBox 360 console doesn't detect any audio or video input. XBox Support claims this will only occur on consoles that do not have both AV and HDMI ports. This problem simply requires you to unplug all of the audio and video cables and plug them back in (the color-coded cables that connect your system to your television). If this does not work, you may need to use a different set of AV cables.
E74
You must send your XBox 360 in for repair if you encounter the E74 error. This problem is indicated with one flashing red light accompanied by a message on the TV screen ending with the error code E74. According to Support.XBox.com, you can request a repair online (see resources). Microsoft covers the cost of E74 repairs within three years of your console's manufacturing date.