Sometimes after rebooting the machine with sql server 2005 standars installed I cannot connect using the sa user and I receive an error when I try to manage the user:
with password = 'yourpwd' unlock,
check_policy = off,
check_expiration = off
After this all works ok.
For details look at http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1673143&SiteID=1