By default, Windows XP creates an Administrator account that is usually hidden from fast user switching unless you go to Safe Mode (F8 on boot). If your computer came preinstalled with XP, this admin account likely has no password, making it easy for someone to boot your computer into safe mode and wreak havok. A simple way to disable this account is to go to Start » Right-click My Computer » Manage » Local Users and Groups » Users. In the right pane you will see all of the users accounts on that computer. To disable the Administrator account right-click it and select Properties. In the window that pops up you will see a check box for Account is disabled, check that.
Alternatively you can give the Administrator account or any user account a password by right-clicking on the account and selecting Set Password.... If the current account you are logged into has administrator privileges, this will reset the password with the password you type in. You do not need to know the old password. However, if you attempt this read the warning that is given to you first, which states that data loss of some items may occur.