I have actually resolved this issue but I am posting it here to help anyone else who may encounter the same problem and hope that someone may shed light on why this happened or how to stop it recurring.

My original post was :-

Citrix Metaframe XP 4.0.4 (with rollup patch pse400w2kr03) running win2003 SP2 server bombs out with a STOP - Fatal system Error C000021a (0x00000005 - Access violation. This only occurs on a server restart and it appears just as you are expecting the login dialog box.

Also installed are the Novell Client 4.91 sp3 (plus all latest patches) and the Zen agent from 6.5 sp2.

Only happens when pointing the ctxginadll to nwgina. (HKLM\software\Windows NT\winlogon).

If this key is changed to "MSGina" then the server boots fine and you can login via windows - the Novell client appears to function fine after that.

Hardware is HP Proliant DL380 G4 with latest HP firmware and I have ran server diagnostics test with no errors.

Server has been running quite merrily for a month with no (known) updates, after reboot this situation arose.

This problem happened two months ago on the same server. Posted calls with Novell and Microsoft without any joy - had to rebuild the server.

The resolution surprised me

I found a "Novell Zenworks" folder (no quotes) under "c:\Documents and settings\" which was date stamped at around the time the problem arose. It also had a subdirectory called "SgrtFG~" which contained almost all of the normal profile files and folders (ntuser.dat was missing, for example.).

After deleting this folder and resetting the registry key back to nwgina.dll, all was well.