You can do what SeanWebster said below, but the overheating it part can be quite dangerous to the other components on the motherboard. I fixed Xbox 360s for over 4 years, and I can't tell you how many came to me fried because of attempting to fix it like that. The E74 error is a GPU error, that usually is caused by a cold solder joint under the GPU. What SeanWebster is talking about resetting it, the overheating actually melts the solder under the chip and fixes the fault, but causes more damage to other components.
What I suggest is this:
Take it apart, and make sure to clean all the dust in the system, remove the heatsinks for both the CPU and GPU, and clean all the thermal paste off with at least 90% alcohol. Once both dies are clean and shiny, get a heat gun, and set it to about 400F, or "low" setting. What you are going to do then is lay the board down on a flat surface, and start heating the board. You want to do this evenly by moving around the heat gun. Make sure you don't go any closer than an inch, I'd say two inches for safety. Do this for about one minute or two. Then, hold the heat gun above the GPU die, two inches above, and heat the die for about 3 minutes total, make sure to move the heatgun in small circles around the die every 30 seconds or so. The GPU die is the one that has the flat heatsink on it, newer models have a small heatpipe coming out of it. Now, LET THE BOARD COOL ON IT'S OWN! After about an hour, come back, apply fresh thermal paste, and put it all back together, turn it on and run the system for about 30 minutes, to make sure it's okay. Then, enjoy.
I always fixed systems with a reflow station, but this works well also, just requires more care.