To recap - I foolishly upgraded my Nvidia driver on Friday (I think it was Friday) - I haven't been able to use the main SL viewer since. At first I would crash within minutes of logging on - after a while I would crash while logging on. I tried going back to earlier versions of the driver but it didn't help. I could use the old RC I have, so I've been working with that for the past few days. With mixed results.
Today I installed an even older version of the Nvidia driver thinking that might help. It didn't. So in desperation I uninstalled SecondLife. I downloaded and installed the main viewer again and redid all my firewall settings.
And yay! it seems to have worked. I hate to be too confident - but I've been working for about an hour and haven't crashed yet. I did have to change some of my graphics settings though - the quality wasn't good enough. But even that hasn't caused problems.
/me crosses fingers