So, you've got a couple options here depending on your scenario. These guys are talking about NRPE_NT. If your windows clients are Public Facing (ie: they have an external IP and a firewall that you control, or you can set up port fowarding to an internal bo:evil:, you can use an app like nsclient++. Nagios service checks query those external IP's, nsclient or nc_net answers, and ta-da, works just like nagios.
If your windows clients DON'T have an external IP, or you/the company doesn't want to punch a hole in the firewall to let nagios query NC_Net or NSClient, Then basically what you're going to have to set up is something running on your client windows servers and sending PASSIVE results back to your nagios server. That means setting up service checks in nagios with "active_checks_enabled 0"
Typically in a linux environment, an application called NSCA sends passive results back to nagios. There is a windows NSCA available now too but i've never used it:
nagiosexchange.org/cgi-bin/p ... 0.html;d=1
I'm sure you can get stuff to pass data to nsca which it can then initiate an internet connection and send the results to your nagios server, but an app called NC_Net will do passive result sending all by itself (it has a built in nsca, or it can also communicate with nsca for windows)
So, check out NC_Net
nagiosexchange.org/cgi-bin/p ... 3.html;d=1
It's been a while since i used it so im not sure if you can be like "check disk every 5 minutes!" and have it go because i had never set it up that way. You might have to VBscript or make some batch files or something that calls nc_net (or nssclient++ and NSCA-win32) every x amount of time.
Hope that helps