I hope some will respond as well. I'm sure someone else has been through this.

Also, I have used exec sp_change_users_login with the 'auto_fix' option to fix my NT authentication logins in the past as well, in this case, it did not work.

I found this article that helped me work around it.. worth bookmarking if you will be going through this drill later!

http://support.microsoft.com/default.aspx?scid=kb;en-us;819261&Product=sql2k

if the link doesn't work - it is KB article 819261

It's just so puzzling!!! This user has the exact same permissions in the Instance on his 'old' domain NT account as he does on his 'AD' domain account and the old one works and the new one doesn't. I can even duplicate the error use MY OWN "old" and "AD" domain accounts.. and I have sysadmin on the Instance!!