When XP is first installed, by default, an administrator account is created
and is hidden. If you have XP Home restart the computer and repeatedly press
the F8 Key until you see the Boot Menu. Select Safe Mode. As XP boots it
will present you with a log on screen. Choose "Administrator" and leave the
password blank. Once at the Desktop you can go into Control Panel->User
accounts and give a new password to your regular account.
If you have XP Pro follow the above, or you can press Ctrl+Alt+Del twice at
the logon screen rather than booting to Safe Mode, and type in Administrator
account you will have to enter the password that was used with this
administrator account when XP was first installed.