I kept getting an installation error with Windows 7 Service Pack 1 with an error code of: 8024200D.
I tried the Windows Update Readiness Tool multiple times, as well as restarted my computer multiple times. None of that helped.
Finally, I found the download page, from where you can download the entire installation package (http://www.microsoft.com/download/en/details.aspx?id=5842). After downloading the almost 1GB package, I was able to finally get SP1 installed successfully.
The installers based on your platform are (you need to validate that you have a genuine copy of Win7):
More info: Steps to take to make the SP1 install go smoothly: http://support.microsoft.com/kb/2505743