It sounds like you created your user account with admin privileges and possibly some extra accounts when you installed XP Pro.
If you create any accounts XP Pro also creates an Admin account by default I think, which would only be accessed in safe mode.
If you installed it yourself then the password should be blank, if you just pressed enter and didnt type a password in, if your pc came with XP preinstalled then they may well have used the password of admin or administrator.
So I would try just logging onto the Admin account by hitting enter or by using admin or administrator and see if any of those work.
As long as you have admin privileges on your user account then you dont need to be able to access the admin account anyway as there wouldnt be anything extra in there to what you already have available through your own user account