Registry cannot load the Hive (SAM) file error... and Windows fails to boot.

Bubsy, have you considered a Repair Install? Check below at Michaels
site for more info.
http://michaelstevenstech.com/XPrepairinstall.htm

Reboot means different things to different people, so please describe
what reboot means to you and what you see on the screen.

When the system was working properly was it set to login
automatically?

Have you seen this article that highlights your exact issue?

http://support.microsoft.com/kb/822705

Let's say your registry is somehow FUBAR. It is not hard to replace
the 5 "important" registry files involved in the startup process with
some older ones long enough to allow you to at least boot and then
decide what to do next.

I am not paying attention to all this try, try, try stuff. You can
try all day, you need to fix.

There is not moving hardware around to other systems. If your system
gets far enough to get that error, then XP is at least trying to load,
but can't read the file.

If the registry files are screwed up, replace them.

I would put all your original hardware back like it was to see the
error, boot the Windows Recovery Console from your genuine XP
installation CD or if you don't have one, make a bootable Recovery
Console CD (easy), then proceed to fix it.

Run chkdsk, etc. all from the RC.

There is really no such thing as a manual system restore. You can
implement a portion of what an actual System Restore does with
KB307545 to get you going again and then do a real System Restore.

You can implement KB307545 from the Recovery Console:

http://support.microsoft.com/kb/307545/

For those that think KB307545 is equal to some kind of "manual system
restore" perform this convincing test:

Create a Restore Point, uninstall something simple like Games. Reboot
and Games are gone. Perform a System Restore, reboot and the all the
games are back and work fine.

Uninstall Games again, reboot and implement KB307545, reboot and see
how your games work now (they don't).

KB307545 is not the same as a System Restore and it is not a manual
system restore. It is to allow you to get your system running long
enough to do a proper System Restore (if it is still needed). See the
often overlooked Step 4.

After running Bitdefenders recovery CD's memory test, I learned that one of
my two memory DIMMs was NG. (each is 256). When I removed it, I was able to
boot the clone disk. But that disk is from 2008 and some very important
changes were made to my system between then and now, the most important of
which was a different antivirus program.

Here is where I'm at now with the Original Disk. The Windows Repair console
did allow me to begin a Repair Install of windows. It checked the files,
copied the new ones, then rebooted checked the installation, then said the
Repair Installa was completed and needed to reboot a final time. At that
point, when it triedto go into the Windows GUI, I got

Stop C000021a Fatal System Error. The Windows Logon Process System Process
Terminated Unexpectedly with a status of 0X0000135 (0X00000000 0X00000000)
The system has been shut down