Here is your Owner's Manual:http://support.dell.com/support/edocs/systems/dim5150/en/om/WD846A02.pdf
Look in the troubleshooting section, and match the color/pattern of your little error lights (1-2-3-4) with the table in the manual.
lol i know it is a late reply but, anyway, i tried that thing you said. the dell information did not help out all (first and last time a buy a dell). it turns out it was the frontal i/o and the motherboard battery the whole time.