I won't give the name of the person who helped or the website it came from, coz they haven't expressly given consent to do so but they have OK'd passing on the fix, which is:-
Vista periodically saves the registry Hives to the regback folder in C:\Windows\System32\config\regback.
If the registry Hives have not been updated you could copy them to C:\Windows\System32\config\.
I would rename the the Hives in the config folder to
Software to Software.BAK
Security to Security.BAK
Default to Default.BAK
SAM to SAM.BAK
Then copy, not move, the registry Hives from the regback folder to the config folder using your rescue USB flash drive. Reboot. Depending on when the Registry Hives in Regback were create and saved then in the d, you may have to do some registry fixes in the malware removal forum but this may get you to a bootable state.
In addition, from the same source,
The files are overwritten by Windows every so often but I don't know what the time interval is. As long as you see that the date on the Hives are earlier than your problem then there may be a good chance at recovery.
I made a backup copy of the four files from the regback folder and saved them in that folder, obviously with a different file name, so that when Vista overwrites the originals I've still got unmolested copies to fall back on should they be needed again.
I don't know if this fix is what the System Restore automates but in my case the System Restore did not work so was not an option. Now knowing the simplicity of this fix I reckon it would be wise to periodically backup the files Software, Security, Default and SAM found in the regback folder just in case.
I hope this helps.