Here are the steps to install Windows 7 Service pack 1. First you need to download System Update Readiness Tool for Windows 7 either 32 bit or b4bit based on what version of windows you installed on your Machine. Here is the link where you can download the above mentioned tool.
why this tool needed?
This tool is being offered because an inconsistency was found in the Windows servicing store which may prevent the successful installation of future updates, service packs, and software. This tool checks your computer for such inconsistencies and tries to resolve issues if found.