Microsoft as and when releases a Service Pack to its Windows and Office Products always pushes the service packs via Windows Update. In order to get the packs via Wndows Update, you need to turn on Windows Update for Windows 7 and Windows Vista, or Automatic Updates for Windows XP, and let Windows notify you when the service packs you need are ready to install. Turning on Automatic Updating is quick and easy, and it saves you time and disk space.
But there are times, when you need to download a service pack manually rather than waiting for Microsoft pushing it through Windows Update for Automatic Update.
The Redmond company has created a "Service Pack Center" on Microsoft.com where you can get the latest service pack for your version of Windows including Windows 7, Windows Vista or Windows XP.
Just head over to the Service Pack Center, where you will find the latest information, downloads, tips and troubleshooting for all the most recent service packs.
"service pack (SP) is a Windows update, often combining previously released updates, that helps make Windows more reliable. Service packs, which are provided free of charge on this page, can include security and performance improvements and support for new types of hardware. Make sure you install the latest service pack to help keep Windows up to date. Service packs take about 30 minutes to install, and you'll need to restart your computer about halfway through the installation."
More Info: Service Pack Center