Is there any way to reset the password on the ADNM?
Unable to obtain password from previous network administrator. So nobody knows what it is. Have full access to the server as administrator.
See ADNM manual, for default password if it is not changed… 
I´m afraid that reset password is feature not implemented, or not for public use. When default password is not changed, ask AVAST support about reset or reinstal ADNM.
Before reinstall you can try backup SQL database (via Maintenance Tool) and after reinstall, recover database…
the default user/password for ADNM is Administrator/admin
big chance its changed…
tho the idea of Paba is nice, i believe that with restoring the database you also restore the users and passwords for
accessing the ADNM interface and thus you still have no good working system BUT… its worth the try on a test machine
edit 02-6-2011: If you have access to the database from avast then you can also manually change the password in the database itself. Find the user that you want to change the password from, type in plaintext the new password and you can login again. After you logged in i believe the password gets encrypted in the database again.