Windows XP (both Home and Professional versions) have a "hidden" administrator account. By default, it does not have a password set. In order to avoid serious security problems, it is best to set a password for this account on your computer. To do so, follow these instructions:
Turn your computer off completely. Wait 15 seconds, then restart it. When your computer manufacturer's logo appears on the screen (Dell, Compaq, HP, etc.), start pressing the F8 key on your keyboard. If you've done this correctly, a text menu should appear. Use the arrow keys on your keyboard to select "Safe Mode," then hit Enter.
The next text menu will present you with an operating system selection. In most cases, you will only see "Microsoft Windows XP." Make sure this is highlighted, then hit Enter.
When Safe Mode has loaded, you should be presented with the Welcome screen. Select "Administrator."
When the Adminstrator account is loaded, you will be presented with a warning message. Click the OK button.
Open the Start menu, then click on Control Panel.
In the Control Panel, open the User Accounts icon.
The User Accounts screen will list several accounts that are present on your machine. Take note that you will likely have one for Administrator, and another one (that you named when setting up your computer initially). Select the Administrator account.
In the Administrator account settings screen, click on the link for "Create a password."
In the password creation window, fill out the requested blanks When you're finished, hit the Create Password button.
You may be asked if you wish to make this account's files private. Click either Yes or No (depending on your preference), then click Finish.
You should be returned to the User Accounts window. Close this window, then restart your computer normally.
NOTE: Make sure that you add account passwords on ALL of your computer's accounts. Click here to find out how.