If you have Windows XP Home Edition and haven't set the Administrator password (very few do that on Home Edition) you can boot inte safe mode and login as Administrator and then reset the password on your user account.
If you have Windows XP Professional and is not running as the original administrator account, try logging in to that account if you can remember the password you set during install (it might be empty). You might have to disable the Windows XP 'Welcome screen' and enable the classic login screen to be able to login as administrator. This can be done in Control Panel, User accounts.
/kollo23