password lost

i have in my lab a server setup by a network administrator and since he has left no one has gain acces to the server and no update of the antivirus has been done. i need to access the server so as to update and maintain it. i don’t want to format the hardrive because the server is being used by alot of users and also on a large network interface. the hardrive is in ntfs and cannot obtain access with a bootup disk. what should i do?