If you are using a Microsoft Windows operating system, go to http://update.microsoft.com/ to check for the latest updates. This service will automatically tell you what updates you need (for Windows, Office, Exchange Server, and SQL Server). If you would like Microsoft to automatically update your system without any user intervention, go to the Control Panel and double-click on the Automatic Updates icon. Be sure the first box is checked to "Keep my computer up to date". Then select the option below to "Automatically download the updates, and install them on the schedule that I specify". Please choose "Every day" from the first drop down and a time in the second drop down when you know your machine will be turned on and connected to the Internet.
If you choose to manually update your computer, you should always accept the patches labeled "Critical Updates and Service Packs". Those are the patches that will protect your computer from exploits. You may also want to install some or all of the patches labeled with your version of Windows (e.g. "Windows XP"). Those are optional. Finally, you may also want to install the patches labeled "Driver Updates", if there are any. Those are also optional.
If you are using a Mac operating system, reference the software update feature for the latest updates. More information can be found at http://www.info.apple.com/usen/security.
If using a Linux or Unix (*nix) based operating system, please check the vendor/provider website for system patches.
Also, visit the websites of other companies whose software you are running on your computer to find any available patching information. Not only will this help make your computer more secure, it can make it more stable.
Keeping your computer up to date is vitally important. New exploits, viruses and worms and being released on the Internet constantly. Many of these malicious programs are designed to take advantage of weaknesses in operating systems and the programs that run on it. In order to protect yourself against these weaknesses, you must patch your computer routinely.