Open a command prompt and type, "net user administrator /active:yes" and then press enter.
When you log out and log back in, you'll have the option of selecting the Administrator account.
If you have tried to install SQL Server 2008 R2 Developer Edition on Windows 7, you have probably spent all day (like me) getting the following error...
"The process does not possess the ‘SeSecurityPrivilege’ privilege which is required for this operation." [More]