try being more specific!
what modchip wud be a good start!!!
anyway u say ur getting a red/green flashing light - if its the light on the modchip then I
assume u have a matrix and then it wud just be an alignment problem...
if so go read the complete tut on matrix chips in the tut section and try realigning it...http://www.xbox-scene.com/tutorials.php
if its a solder mod then recheck all your connections and make sure that there's no stray
solder elsewhere on the board that may be shorting things out........
basically check again, if u can't find anything wrong, remove modchip, see if box boots up
fine and then try again