Since it's the Windows logon system that's failing, and if you're sure that regular Windows safe mode doesn't work (which it should) try bypassing that with safe mode; one of the safe mode options accessed through F8 during boot is "Safe mode with command prompt" - that version of safe mode doesn't load the GUI shell and so shouldn't invoke a GINA. Or, from a Windows install CD, boot to the recovery console which is morally equivalent.
That'll give you a command prompt which will let you copy MSGINA.DLL - that's the standard Windows login provider - to the location Windows expects to find GINASTUB.DLL, which is probably the easiest way to get into the GUI world where you can run the registry editor.
To stop Windows trying to use GINASTUB.DLL at all, you need to do a registry edit: http://support.microsoft.com/kb/164486 - it's reasonably easy to do this from the safe mode command prompt using the REG.EXE command-line utility (REG EXPORT the winlogon subkey, edit that with EDIT.COM, REG IMPORT to apply the changed .REG file) try copying the MSGINA.DLL file first since you're probably more comfortable with the GUI registry editor.