It's possible that you have RAM problems. If you have more than one RAM module in your system, try removing different modules and seeing if you can boot. If you can boot with one of them removed, you've found the bad module.
It's also possible that your CPU is overheating. If this is the case and you test too much, you run the risk of frying your CPU.
I have the same problem here...spoke with Dell who had me do a bunch of plugging/unplugging of everything (modem, ram, power cables, cpu, etc). They suggested replacing the power supply and cpu. For that, I think I can find a new 2400 and just swap out the hard drive and cd burners.
Let me know if you find out anything...I am new to this site. It seems pretty sweet though.
More than likely, you're looking at a bad CPU or Motherboard.
Did you disconnect all drive power cables and data cables, too? You can try clearing the CMOS jumper on the system, too. Short of that, you're probably looking at a dead motherboard or CPU, like I said.
yeah, i figured as much. the power supply has 5v on pins 9 and 14 if i remember correctly. even with the ac power unplugged from the ps it has 5v on those 2 pins.
surprisingly, dell has pretty competitive prices on replacement parts ($30 for a ps, $140 for 2.4 gig celeron, and $80 for mobo). my friend may opt for a new system and then swap out the hard drive and grab the extra ram out of the dead machine.
the dell site has some nice features for troubleshooting, except they dont have much detail on troubleshooting power supplies or mobos.
If you are talking about having 5v come through on thos pins when the power supply was connected to the an AC line but the system was off, that's normal. Since the ATX spec, power supplies always give power to the motherboard on certain pins in order to support the motherboard turning the system on through the power switch being activated or some wake-on ring type peripherals.
I've done a few laptop and desktop repairs and I found out that the power supply units (PSU) for the desktop computers have built in safety features that will actually stop the flow of electricity to the motherboard if a bridge is detected (basically a short in the board). Usually when your board tries to power up and you see the CPU fan spin for a few seconds and then shuts off; that usually means theres a short in the board somewhere.
Common bridges from older computers (past 5 years) include worn out capacitors or transisters (buldged shape or oxidized brown residue around problem areas are common signs). Solder can also cause a bridge; you see, over time through many cycles of heating and cooling, the solder joint can actually go bad (many people call it a "cold solder joint").
If you want to spend the time and energy locating the short in your board they do sell socket testers which can determine if all your IDE, RAM, video, and ethernet sockets are good or bad. You can also buy a digital continuity tester which you can place over each solder joint to determine if there is a bridge between any two points (otherwise known as an electrical short). Once located use a desoldering braid and desoldering sucker in conjunction with flux paste to remove the old solder. Then place brand new 60/40 solder into the joint.
Due to the large number of points to be tested you'll probably not want to undertake this task, instead there are companies that specialize in motherboard level repairs. Simply google "board level repairs" and you should come up with some companies that you can ship your computer too, generally they don't charge to diagnose your computer and you'll only have to pay if they are successful in fixing your mobo.