It seems people updating to the current NVIDIA driver are having problems logging in. They crash on viewer launch.
As of April 6, there is an NVIDIA driver version 381.65. This is the one people have a problem with. The previous version 378.92, the one I am running, does not have the problem. So, some people are rolling back to 378.92.
Whirly Fizzle has posted in the SL Forum that the problem has been tracked to NVIDIA’s Shadowplay/Share feature. Turn it off and things work.
We are early in this problem fix cycle. So, we may learn more soon.