>I have tried to create a new user several times, and each time I get the error
>
> error 15120 unexpected error during password validation.
>
>This not only crashed SQL Server, but also crashes windows!! (2008R2)
>
>All I am doing is trying to create a new user (SQL validation with SQL sysadmin privileges).
>
>Wow! this seems like a huge glaring bug, but I don't find anything in MS about it.
>
>
>Any help would be greatly appreciated!

What is your SQL Server version exactly (select @@Version)?

Also, did you try to script the action and do this from a script?