Ok, I got an error when I turn the computer on. It reads:

Drive 1 not found: Serial ATA, SATA-1
Drive 2 not found: Serial ATA, SATA-2
Drive 3 not found: Serial ATA, SATA-3
Strike the F1 key to cintunem F2 to run the setup utility

I press F1 and I tried all the options there (safe mode, normally)

Then I get another error, it reads:

File Missing or Corrupt: C:\windows\system32\config\system
You can attempt to repair by starting Windows Setup using the Original CD. Select 'r' at the first screen.

I do that, and it reads:

Setup did not find any hard drives installed in your computer.

Any ideas?

Is this a new problem?

Did it work previously? If so what changes were made?

Did you build this computer? Are you familiar working inside it?

Yes this is a new problem.
It was working fine before.
I don't know of any changes.
No this is a Dell
Yes very familiar with the insides of the computer. I opened it up, and unplugged the Hard Drive power and the cable. And that didn't work. I thought maybe a loose connection. However it is a SATA cable, now I have never worked with these cables before, are they know to go bad?

Are there 4 SATA ports or only 3? How many drives are installed? Usually there would be a Drive 0.

Try the plugs again. You can also try different SATA ports but bear in mind if you do you will need to change the BIOS Boot Menu to the proper port. By default it is probably Drive 0.

The hard drive is the only drive that has the SATA. There are 2 CD/DVD Roms, but they are IDE. There are 2 SATA connections on the motherboard.

Well it is strange that 3 are listed (perhaps one is exterior).

It is also strange that bypassing that error (F1) finds Safe Mode options meaning it does find the hard drive.

I would try the other SATA port and also verify in your BIOS Setup that the drive is or is not detected.

I know Dell has diagnostics from within Windows but don't know if they have any BIOS level diagnostics such as hard drive verification.

What is the exact Dell model and number?

Could also try to set the BIOS Defaults.

Could try a different cable but there isn't much that can go wrong there.

Then there is the possibility that the drive has mechanically failed.

Again check connections. Is there a power LED for the hard drive? If so does it go on?

Sorry made a mistake. There are 4 SATA Ports on the MB. 0-3. However 0 is blue, and the cable from the HD is blue also, so currently the cable in plugged into the blue 0 port.

The model number is: Dell Dimmension 8400.
Bios Version A03.

I do not see an LED light on the hard drive. However I didn't take the entire HD out.

Any ideas?

When the comp starts I press F12 and the Boot Device Menu appears. I ran the Hard Drive Diahnostics and is passed on Drive 0. So maybe the hard drive isn't bad?