XP's security does not allow passwords to be bypassed. If you don't know what it is your screwed.
But, if you remember the "Administrator" password you set during the setup process, you can use that to recover it. At the welcome screen hold CTRL+ALT and hit DEL twice. A different login screen will appear. Enter "Administrator" as the username and the password is the one set during setup. Log in and access the User Accounts control panel and change the main user's password
If you do not know the "Administrator" password either, Fdisk, Format, Reinstall.