I'm the one who is thick, sorry...sometimes I don't read titles and sometimes I just don't read at all...my apologies.
Your comment about "running a test" was all that I remembered when I replied. Speaking of which..."uncorrectable data error or media is write protected"...how did you get that interpretation of your error message?
STOP 21A errors...can be any number of things, including malware. Following is from http://www.aumha.org/a/stop.htm:
This occurs when Windows switches into kernel mode and a user-mode subsystem, such as Winlogon or the Client Server Runtime Subsystem (CSRSS), is compromised. Security can no longer be guaranteed
. Because Win XP canít run without Winlogon or CSRSS, this is one of the few situations where the failure of a user-mode service can cause the system to stop responding. This Stop message also can occur as a result of malware infestation or when the computer is restarted after a system administrator has modified permissions so that the SYSTEM account no longer has adequate permissions to access system files and folders."
This is pretty much in line with what I have seen for many of the 21A STOP errors carrying a status of 0xC0000005.
I will (now that I realize your system is not bootable) ask one of the BC Malware Team members to take a look and offer some suggestions. They can offer you better assistance and guidance.
Please be patient