Unable to login sql server 2008 on windows server 2008

You wont be able to login into the account i.e. the sql server account as the login will fail when you try to login into the sql server 2008 after installing the active directory.

It is recommended that you do no install active directory on the server on which you have installed SQL Server 2008. This is not a problem with windows server 2003.

If you install sql server on a windows server 2003 domain controller you will still be able to login into the sql server but is not the case with the sql server 2008 bcz while logging into the sql server the sql server authentication system looks up for the system on which you installed the sql server ... rather then looking into the active directory.