persistant ntdetect error - help!

The problem originated from a faulty secondary drive. I removed it and
tried to replace it; thats when everything started. For some reason
the drives would not function together and Windows would not boot. I
checked the master/slave relationships, tried cable select, but it
wouldn't boot. Since I was having problems with slow PC performance,
and I recently backed up my files,I decided that since I was reparing
one thing, why not clear the machine and reinstall windows?

The format and file copy itself went smoothly, but when the computer
restarted, it passed the DELL logo, then went to a blank screen,
saying "NTDETECT failed.", then the computer rebooted, and the process
repeated itself.

Here I called Dell, and following the instructions, I ran an fdisk and
a debug. However, the problem still persisted.

At this point I had a list of potential causes. 1) The hard drive file
system was faulty 2) the hard drive was physically damaged 3) the
installation disc was faulty 4) the BIOS had a problem.

The hard drive was not the cause, nor was the isntallation disc. I
transferred it to my brother's computer (exactly the same as my own),
and the installation finished successfully. However, when I brought it
back to my own machine, the same "NTDETECT failed" message appeared.

I emailed DELL and was instructed to flash the BIOS. I did so and the
problem persisted.  Then I was instructed to replace the ntdetect.com
file, which I did, with the same results.

I tried one more experiment. I backed up my brother's C: drive
and transferred it to my own to see if it would boot. I got the same
NTDETECT failed. I tried to reinstall windows on the hard drive, but
it resulted in the same ntdetect error.

Does anybody know what I should do?

I have a Dimension 4300 and I use Windows XP, not NT (for some reason ntdetect seems to be associated only with nt)

Thanks