the reason could be that as security measure the previous user must have named the administrator account to something else and might have created a local user account named administrator, that's possible.
you can either do a complete repair, booting from the Win NT WKS CD, this will give you the system with a fresh OS, with administrative rights, from here you can upgrade to Win2K. before doing this you'll need to uninstall all the programs, i guess you won't be able to uninstall some of the programs since you don't have admin rights.
so delete those from program files or whatever folders they are in before installing Win 2K. you can safely do this b'cos Win NT is a fresh one so all the system files related to these programs are deleted anyway and they need to be reinstalled for them to work.
this should help.