Nagios Support Forum

I believe it would be in the best interest of time as well as sanity to import the old config files to a completely new install, this way you know where you sit, and have the ability to configure it to your liking. Not to mention that it will be a heck of a lot faster than troubleshooting the old system.

Here are the instructions for installing from source:

http://assets.nagios.com/downloads/nagi ... Source.pdf

And backing up and restoring the old server's configs:

Create a directory for the backup and tar up the configs:
To restore the configs simply unzip the /usr/local/nagios/etc contents to the new system.