the file is named passwd in /etc/
Have you got a backup of it with a blank password? If not I can sort you one out.
It's not a simple text file that you can just edit. It can be done but I can't remember how offhand. It's easier to just copy a new file over.
EDIT: I googled it lol
If you open the passwd file with notepad++ you have to edit the line that begins 'root'
It should looks something like this:
root:x:0:0:root:/root:/bin/bash
Delete the "x" after "root:". It's important to leave the colons.
You may have to reboot but next time you telnet in you will have a blank password.
Create a new one by typing passwd