Hi Ian
Although your problem seems to now be sorted I thought I'd offer a bit of info.
If your windows update is set on automatic, and you have recently downloaded an update, which you may not be aware that you have, especially SP2, then this could be your problem.
SP2 has its own in built firewall, I don't know what the default settings are but they may be enough tostop your password thing.
The point is that although you have rolled back to a previous configuration the automatic updates will be downloaded again, and again, and again etc.
If it is firewall then you need to reduce the security settings.
Also, be aware that your restore point may have gone back further than you antisipate. If you have recently installed software then you may need to re-install it.
Chris