Helping people with computers... one answer at a time.
Windows Update: the web site, the application and the automatic updating facility build into Windows to keep it safe, stable and secure.
Support from Microsoft for SP2 has ended. SP3 has critical security patches that need to be installed on your computer.
Most often, you'll want to take the updates offered by Windows Update. If you want to delay or completely avoid an update, I'll show you how to hide it.