It should have also given you a state code as well; searching the combination will show you exactly what the issue is. Are you sure you enabled mixed mode authentication when installing? This article shows you how to enable it: SQL 2008 R2: Enabling Mixed Mode Authentication
This code will verify whether it has been configured for mixed mode authentication already:
SELECT CASE SERVERPROPERTY('IsIntegratedSecurityOnly')
WHEN 1 THEN 'Windows Authentication'
WHEN 0 THEN 'Windows and SQL Server Authentication'
END as [Authentication Mode]