Securing Microsoft Windows XP
Windows Update
Setting up Windows XP to automatically update is relatively simple! When you first install XP it should ask you about automatic updates. If for some reason it doesn't, or you don't know if you have automatic updates activated follow these steps. First click on the Start Menu, open up the Control Panel, and from there, select Automatic Updates.
Quick Reference: Start Menu → Control Panel → Automatic Updates

Once you see this screen choose the radio button for "Check for Updates Automatically." Have your
computer check for updates once a week, and be sure to enter a date and time
when your computer will be turned on and connected to the Internet. You might want to set the time when you wouldn't be sitting at your computer, while the updates are for the most part unintrusive they can slow performance.
It's also a good idea to know how to check for updates manually, since Microsoft releases updates fairly frequently. You can always open up Internet Explorer, choose the Tools menu, and select Windows Update from there. Then follow the instructions to check for and download new updates!
Quick Reference: Internet Explorer → Tools → Windows Update
Turn on the Firewall
By this point, your computer should be up to date, running the latest and greatest, which means Windows XP SP2 (Service Pack 2). It's a snap to turn on the Windows XP SP2 Firewall, so lets do that now. Go to the Start Menu, open up the Control Panel, and choose Windows Firewall. You should get a screen similar to the one pictured below.
Quick Reference: Start Menu → Control Panel → Windows Firewall

Now select the radio button that says "On". And we're done!
Account Management
We're not going to go too in depth on this subject, but it's good to know exactly who can access your computer. When you use Windows XP, you are probably using an account that you set up when you first started using your computer, or the last time you installed Windows XP. Windows XP usually generates a few other accounts automatically, that you might not even realize are there, including an Administrator account with NO PASSWORD - which is a known fact that many of the more nefarious viruses exploit.
To take a look at the user accounts on your computer, choose the Start Menu, open the Control Panel, and choose User Accounts. You should get a screen similar to the one below:
Quick Reference: Start Menu → Control Panel → User Accounts

This is a list of all of the User Accounts on your computer. Take a minute to
remove accounts you know are not being used. Check and make sure all accounts
have a good password. For guidelines for creating a password read common sense security. It is especially important to give every administrator a password, as these accounts have the most freedom on your system.