Slipstream SP1 into Windows Server 2008 R2 installation files
It's messy, so it's usually easier to get a copy of Server 2008 R2 w/ SP1.
You can do it manually. The direction for Win7 are the same.
Just download it directly:
http://www.shifnet.com/blog/technology/download-windows-server-2008-r2-sp1-official-iso-180-days-free-trial.html