ipsentry.com – cheap, runs on Windows, only one license needed to monitor any number of systems.
Word of mouth, I’ve heard time and again that SpiceWorks is a good starting point.
I have read a lot too about GroundWork open source. Nagios is one of the other recognized tools for network monitoring. There are tons of plugins and books about Nagios.
Something a little easier on the Windows side is a program called “The Dude”. Rather like a simple free version of What’sUp Gold.