I have a dell optiplex gx 520 that lights up, I can hear the fans and drives but it won't come on. The power light is solid yellow and the four lights on the bottom show 1 & 3 green. The CD drive won't even open. I removed the memory and reseated them.
Lights 1 and 3 means that there's an issue with the memory configuration. So make sure that all the RAM modules are in the proper configuration, and if you recently installed any RAM, try taking it out to see if things function. You can also try taking all the RAM out to see if the error codes change.
If no amount of reconfiguring the RAM helps, then you're probably looking at a bad motherboard.