Slipstreaming Windows service packs used to be a simple process with earlier versions - just run the EXE with a special command-line switch. I can't seem to find such a switch with Windows Server 2008 R2 service pack 1. How do I slipstream it? TechNet mentions "integrated installation", so it's possible, but it doesn't seem to have any details on how to actually do it!
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: