I have a dell dimension 8400. when i turn it on the cpu cooling fan comes on and sounds like it wants to take off. Nothing else happens. The monitor stays black.  
Further to this problem.
I checked the power supply with a multimeter and it seems to be good. All the recommended voltages were where they should  have been.  
The four lights on the back A B C D are all off.  
The 3 lights on the keyboard flash on and off when i turn it on.
Can someone out there please help.

 
You might try mounting a different drive in C:, then booting from your install CD.  If you get the same, likely some hardware problem.

I couldnt do that. I cannot even get to the CMOS menu as there is nothing on the monitor. As I said in the beginning all that happens is that the CPU cooling fan works nothing else

Out of warranty?  Time for the repair shop.

 
Let me guess - the light on the power button also stays a steady yellow/amber?  If so, your system is showing the EXACT same symptoms as my 8400 just started showing on Monday.  Usually, if it was the PSU, the power light would be blinking yellow/amber instead of being steady on.  After doing some reading at the Dell forums, I'm pretty sure it's the mobo that's died, so since mine is out of warranty, I went ahead and ordered an OEM replacement from a 3rd party and it should arrive tomorrow.  
 
If I have time before I have to go to work, I'll install it, see if it takes care of the problem, and post here with the results.

Thanks for answering my problem. Will watch for your answer.

Well, unfortunately, replacing the mobo didn't seem to work.  I was cautious to the point that after putting in the new mobo, the only 2 things I put on it before powering up were the CPU and the CPU fan.  The result was the same - the power LED was steady yellow/amber, the CPU fan spooled up to hyperspeed, no HD activity or video output, and the 4 status LED's on the back of the case did not light up at all.  So, I didn't bother connecting anything else.
 
I also purchased a new power supply (470W Silencer 470) and tried that with the same results.
 
I think the only other culprit could be the CPU and, at this point, I'm undecided if I'm going to bother replacing that.  I might just buy a barebones system and move my peripherals over instead.   :?

Thats bad news. I was hoping for your sake and mine that the new mobo would have been the fix. I might follow your lead and go via the barebones idea.

Try putting in a video card from another computer and disabling onboard video, if that is what you have.

Dont you have to do that in Cmos. Cannot get to Cmos as nothing comes on the screen.

I've had a similar issue to yours. I noticed one morning that my machine's cd lights, power etc were all blinking.  
 
I unplugged the machine and plugged it back in, when I did, the fan was superspeed, and smoke came out the back, and there was a burnt smell.  After reading Dell's forums,  several other people had the same issue and noticed their MB p2 connector ( next to the processor is burnt, and also the power supply connector that plugs in.
 
I've replaced MB and PSU, but now the diagnostics show a bad cpu.
 
I believe dell has a serious issue with this connector, but their tech support will not escalate this.  I am concerned about the safety of this MB/PSU. My computer didn't catch fire,  but I don't know if that is because I was there to unplug it.  
 
Please check your p2 connector to see if your connector is also burnt.  I'm trying to get someone at dell to look into this more closely, so that this doesn't seriously harm someone or their property.
 
thanks, Patrick

Just for additional info, I did not have any indications of overheating or smoke or any other problems.  I had just started playing a game and everything froze on the screen and since nothing else was working, I powered off.  I turned it back on and it did boot up and I was able to log onto XP, but just as it got to the desktop, it froze again.  I turned it off and back on, but this time the screen stayed dark, the CPU fan went into hyperspeed, and power button stayed steady yellow/amber and that was it - no HD activity and the 4 status LED's on the back of the case never came on.
 
The very first thing I did was remove the the Radeon X800 XL card I'd been using for over a year and put back in the X300 that came with the system, but that obviously didn't work.  I then started removing/disconnecting stuff one-by-one, trying to power up in between each time, and obviously that didn't work either.  That's when I started checking the forums and thought that replacing the mobo would work.
 
In any case, I've since inspected the original mobo and there aren't any outward physical signs of anything being burnt out.  The capacitors aren't bulging and there aren't any dark spots on the back side of the board.
 
I may have a line on getting a functioning P4 540 (3.2 GHz) CPU from someone to at least see if that works in the new mobo, but I'm not sure yet.  Of course, if I'm able to get it, I'll post the results.

Just an update.  It turns out my problem was the CPU.  I was able to borrow a P4 530 (3.0 GHz) and placed it in with the "new" mobo I had purchased and the computer fired up fine.  I took a chance (since it was a borrowed CPU) and put the CPU in the original mobo and it also worked there.
 
So, I went ahead and bought a P4 541 (3.2 GHz with EM64T technology) and I've put that it in the original mobo and everything is working fine.  I've also went ahead and replaced the original 350W PSU with the Silencer 470W that I had, though that required some modification to the rear of the tower to accomodate the AC outlet which is below the opening for the OEM PSU.
 
Hope you get your problem fixed.
 
(Edited to change MHz to GHz - sort of a big difference...oops)

Glad to hear that you got it working. I havent done anything yet but will try a CPU as soon as I can,

I bought a USED Pentium 4 3.2 Ghz 541 800mhz SL8J2 LGA 775 CPU and it didnt change the situation.
I then bought a Gigabyte MOBO GA-81865GME-775-RH  ( to upgrade my own PC from a socket 478 to a 775) and that way i could test the CPU i had bought and it worked for a couple of hours. I then put the original Dell CPU into the new MOBO and have been running it for the last 12 hours straight. Nothing wrong with the original Dell CPU.
Looks like it must be the Dell MOBO.

If your able to do all that with hardware, why are you buying Dells?
 
Save the headache and just build your own.

I did'nt buy a dell, it's my eldest daughters, I would'nt have one if they gave it to me, just trying to help her out of a mess.I told her to get rid of it, I guess she will when she has some spare cash.
regards Henry

Well the problem has been solved. I found a MOBO on ebay for  
AU$33.08 and fixed it.  Thanks for all who tried to help.

very strange..I just setup my dell 8400 again and had this exact same problem.. I stopped using this dell about a week ago when the parts came for my new computer. This computer was working perfectly fine, no freezes, lockups or random reboots, for about 2 years..then today, dead

IV'E HAD THIS PROBLEM LATE DECEMBER 2006 - IT'S THE POWER SUPPLY DELL CHECKED EVERYTHIN THEY SAID EVERYTHINGS FINE, THEY CHANGED THE POWER SUPPLY IN THE SYSTEM AN NOW ITS FINE AN THIS WAS HAPPENING FOR OVER A YEAR. THANKS.