To run SQL Server 2008 on Windows Server 2008 R2, you need to install SQL Server 2008 SP1. You can either install this after the fact, or create a slipstreamed installation point.
See Microsoft Support: How to update or slipstream an installation of SQL Server 2008
I use a slipstreamed installation point and run setup.exe from the command line - this is much faster, since you don't have to click through the setup wizard or wait for the 'Setup Support files' to install.
Just open an elevated command prompt and run this command to install SQL, Analysis Services, Reporting Services, SQL Management Studio and BIDS in about 15 minutes: