The default password for the Administrator account when Windows XP Pro is initially setup is null, i.e., no password. Easy for someone to guess the name to use (Administrator) and the password (no password needed).
So there are two things to do here to help make your computer more secure -- change the name of the administrator account to something other than Administrator, and give that account a strong password. You will need to be logged-in with Administrator privileges to change these.
- Open the Computer Management module (Start -> Control Panel -> Administrative Tools -> Computer Management). If you have trouble opening the Computer Management module just go to Start -> Help and Support and search for Computer Management which will explain what else it is used for and how to get to it.
- Expand System Tools and then expand Local Users and Groups.
- Select Users.
- Locate the Administrator account (notice the description says "Built-in account for administering the computer/domain")
- Right click on the Administrator account and choose Rename. Name the account to something other than Administrator that would be hard to guess, such as "xAdmin%".
- After the account has been renamed, right click again and choose Set Password. You will be warned that some data might be lost -- you can click Help to see more information about what types of information are affected. Hopefully you don't log on as the user Administrator, so you shouldn't be affected (and if you are logged on as Administrator, you can still change the password from User Accounts in Control Panel). Change the password to a "strong" password. A strong password should be at least 8 characters long and contain a combination of upper and lowercase letters, numbers, and other symbols such as !@# etc.
You've just made your Windows XP Pro computer more difficult to break into. Might as well take the time to change your own password to a strong password while you're at it!