In short, you can't - And with good reason. For more information on possible legitimate ways of changing or recovering your password, here's the Microsoft Knowledgebase Article on it:

How to log on to Windows XP if you forget your password or if your password expires

To log on to XP, you need a valid User Account - That means a username and password. There are other means of recovering passwords, though I won't discuss them any further here for obvious reasons.