Nagios – How to Monitor Windows Host

I am not sure about the CPU monitoring, but you should also look at getting environmental monitoring in whatever space you are putting this server. It will allow you to have a better idea of what is going on in the space, and have more checks other than just CPU temp

http://nagios.org/products/environmental/

Try to do the following if you can:

• Test the authentication backend - does your vpn authenticate against radius? if so, setup nagios with the radius plugin.
• Check for gaps in VPN login times. If you are running a busy vpn server, most likely you see a login at least once every 10 minutes during business hours. I have a custom nagios plugin that queries the radius accounting database, and checks to see if anyone has successfully logged in in the past 10 minutes.

The above 2 'extra' checks have been invaluable.