This guide was written by me for Zeropaid.com several years ago:
http://www.zeropaid.com/news/5412/Automating+Computer+Maintenance
I put this here mainly for my own quick reference. If you want a more verbose version, go here:
New vulnerabilities and flaws are found in Windows each and every day. Fortunately Microsoft releases patches occasionally to close up some of these holes, but it's up to you, the user, to make sure that they get installed one way or another. These patches are growing increasingly important as "malicious computer specialists" are finding ways to exploit the holes more quickly.