why is SQL server 2008 not installing properly?

I am installing SQL server 2008 for the first time and I am having difficulies with the Server Setup...

I have installed SQL server 2008 on another PC and it works perfect...

Why?

[Error Message]
The credentials you provided for the SQL Server Agent service are invalid. To continue, provide a valid account and password for the SQL Server Agent service.

[Details]
Microsoft.SqlServer.Configuration.Agent.InputValidationException: The credentials you provided for the SQL Server Agent service are invalid. To continue, provide a valid account and password for the SQL Server Agent service.
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