did they send it back to you still malfunctioning or was the repair successful and now you have the same problem again?
What you can try is a hardware reset - thats what they call it at Dell. Plug out every peripherals, optical drive if possible, pcmcia cards, even the memory modules, batteries, hdd and unplug the power supply as well - literally everything you can plug out. then press the power on button and hold it for like 20 seconds. this should then pull all power out of the "circuits" and kinda reset the computer. after that plug in all you plugged out before.
well, this is what I would try....
I wouldn't suggestion to start repairing it yourself but I would use the warranty which should apply for a 5 months old computer. If you did not cause the failure you will get it repaired even if it takes a while.
Many vendors even offer a complete replacment after a certain amount of repair tries.
good luck!
mscrx