Windows 7 Service Pack 1 installation trouble

My computer is set to use automatic updates from Windows Update. So when SP1 for Windows 7 came out, I didn't even think about installing it… Windows Update will take care of it for me. But sadly I was mistaking. After two months I realized that the service pack had not been installed. So I downloaded the service pack from Microsoft Download Center, and tried to install it. But I got an error: "