Some errors never got old. I have seen many new DBA or Developers struggling with this errors.
Error: 18452 Login failed for user ‘(null)’. The user is not associated with a trusted SQL Server connection.
Change the Authentication Mode of the SQL server from “Windows Authentication Mode (Windows Authentication)”
to “Mixed Mode (Windows Authentication and SQL Server Authentication)”.
Run following script in SQL Analyzer to change the authentication
LOGIN sa ENABLE
ALTER LOGIN sa WITH PASSWORD = '<password>'
In Object Explorer, expand Security, expand Logins, right-click sa, and then click Properties. On the General page, you may have to create and confirm a password for the sa login. On the Status page, in the Login section, click Enabled, and then click OK.