I am at my wits end not being able to get past the Windows XP loading bar, which suddenly freezes after a few seconds. I tried Booting in Safe Mode but received a message that read
STOP: c000021a {FATAL} System Error: I ran a Hard Drive Diagnostic and that showed "FAIL" - RETURN CODE 7; and a host of other checks including CHKDSK, cleaning memory cards, etc., all coming up empty. I attempted to reload Windows XP, when I got to the install screen it said "unknown disk, there is no disk in this drive". I have a Dell Dimension 9100 Bios Ver A02, with Intel Pentium 4CPU @ 3.00GHz. Any workable suggestions??

That is not a hardware failure. Winlogon or CSRSS crash causes such an error to be displayed.
Mismatched system files have been installed.
A Service Pack installation has failed.
A backup program that is used to restore a hard disk did not correctly restore files that may have been in use.
An incompatible third-party program has been installed.
+ Memory or Hard drive error might cause the installation issue. Disk may be corrupted. You have to try running check disk with parameters again.

Hi Gengis

First you might want to download this manual for your pc:

The error "unknown disk, there is no disk in this drive" can be caused by missing SATA drivers on the XP install cd. (if bios is set to use "AHCI" under drive operation)

See page 107-110 in the manual about the bios settings.

You should make sure that the "SATA Operation" setting is set to "RAID Autodetect/ATA"

If you after that can't install XP, then you probably have a hardware error of some kind.
It can be a defective harddrive, a bad harddrive controller (motherboard problem), or a bad connection between the two.

Let's hope for the first mentioned :)

If all else fails try buying a new hard drive and reinstalling windows to that or borrowing a working hard drive and booting to that.