SQL Server 2008 can't login with newly created user

I'm using using Windows Vista and I'm having trouble logging in with a newly created user.

1. I open SQL Server Management Studio.
2. I create a new Login by right-clicking on Security->Logins.
   Check: SQL Server Authentication
   Login name: tester
   Password: test
   Click OK
3. I added this user to User Mapping to my database of choice.
4. Click File -> Connect Object Explorer, select SQL Server Authentication and enter tester/test and click Connect.

I get an error:

Login failed for user 'tester'. (Microsoft SQL Server, Error: 18456" 
with Severity = 14 and State = 1.

What causes this error and how do I login with my user?