Windows XP, Windows Vista, and Windows 7 make it easy to add additional users to your computer. The extra users can create and save their own files, but (if you choose the settings correctly) they won't be able to read or modify the files that you've saved, and they won't be able to change system settings.

This also offers additional protection against your computer being infected with a virus. Generally the best protection against viruses is to install an anti-virus program (don't get hung up on the differences between them; anything is better than nothing). However, the nice thing about having multiple users on the computer is that if a non-administrator user does accidentally get infected with a virus, it will infect only their files and settings -- it won't affect the administrator's account or the system as a whole. (Basically, if a user accidentally gets infected with a virus, the virus runs "as" that user -- meaning, the virus only has the privileges of that particular user. If that user doesn't have privileges to access confidential files or change system settings, then the virus that infected them, can't do those things either.)

Before you add additional users to the computer, you should set a password for the main administrator account on the computer [link to previously published article on how to do this]. Even if your computer always starts up automatically without ever asking you to "sign in", there still is a "main administrator account" that you're signed in under, whenever you use Windows. That article shows you how to set a password for that account. (Obviously, it offers no security protection to set up extra accounts on the machine for other users to use, if anybody can still sign in as the main administrator account without entering a password!)

Also, if one of the reasons that you're creating secondary accounts on the machine is to prevent other users from being able to access your confidential files, then when you're following the instructions in that article to set a password, when Windows asks you "Do you want to make your files and folders private?", be sure to say "Yes".

Once you have created a password for the main administrator account, you can set up secondary accounts on the same machine. To do this, go to Control Panel and pick User Accounts:

Then pick "Create a new account":

Enter a name for the new account, and hit Next:

The next screen asks you to "Pick an account type". If the reason you're adding secondary users to the system is primarily so that those users can use the computer without the possibility of them damaging your important files, or changing system settings by accident, then choose account type "Limited":

The screen in Windows XP warns you that "programs designed prior to Windows XP or Windows 2000 might not work properly with limited accounts". However, as of 2009, this is very unlikely to be a problem with most programs in common use today, so you shouldn't worry about this.

Then click "Create Account" and the account will be created. Next time you log out of Windows, the account you just created will be listed.

You may also want to set a password for the new account you just created. You should do this, for example, if other people have physical access to the computer, and you don't want anyone to be able to use the computer at all unless you've created an account for them. To set a password for the new account, click the account name:

When asked "What do you want to change about the account?", click "Create a password":

When you do this, you get a screen with a rather ominous warning that if you create a password for the user, they "will lose all EFS-encrypted files, personal certificates, and stored passwords for Web sites or network resources":

If you just created the new user, then these warnings would not apply anyway. And even if you're setting a password for a user who is not a new user, in most cases you still don't need to worry about these warnings. (When the warning says you will "lose stored passwords for Web sites", for example, that only means that when you log in to Hotmail, and you had selected the option for Internet Explorer to fill in your password for you automatically, it won't fill it in for you automatically next time you visit the site. It does not mean that you will lose your Hotmail account!)

Other than that, setting a password for the secondary user is basically the same as setting a password for your own account [see previous article]. After you have set a password for that user, the next time they sign in to Windows, they will be required to enter that password in order to log in.

Bennett Haselton is a technology and political blogger who runs a site about how to bypass SmartFilter and other sought-after information.