To Enter Safe Mode re-boot the computer and keep tapping F8 till you get the boot menu. One of the options will be Safe Mode. Select it and you will end up in Safe Mode. There you will find all accounts with Administrative privileges including one called Administrator. This account cannot be deleted nor can it be disabled. It is only available from Safe Mode and safety net to ensure that there is always at least one account with Administrator privileges regardless of what the user might do. It is impossible to delete all administrator access to Windows XP.
Select it and when it prompts for a password enter what you think is the password, If it doesn't work try entering a blank password as it is often left blank when Windows is set up. If that fails you need to speak to whoever set the computer up and ask them what the password is.