After installing SQL Server 2005 SP3 + CU4 on a Named Instance running on Windows Cluster, authentication mode changes to Windows Only from Mixed Authentication. This was an issue posted by someone yesterday.
About 4 months back, I remember facing the same issue when we had installed CU1 on a SQL Server 2005 (SP3) Named Instance running in Clustered environment. After installing the CU, the nodes were rebooted. When the nodes came back online, all the SQL Server Logins started failing to authenticate. On checking the settings, the authentication mode had changed to Windows Only from the earlier setting of Mixed Authentication. Then the authentication mode was changed back to Mixed Authentication manually.
This is a bug in SQL Server 2005 which is closed as “Won’t fix”. After installing a hotfix on SQL Server 2005 Named Instance on Cluster, ensure that the Authentication Mode has not changed.