System Monitoring tools

I need to find a free, safe open source system monitoring tool that I can use in our lab environment. I was looking at Nagios but saw that it was for *NIX machines and my lab is 99% Windows Server build. Can I still use this or do I have to have a *NIX box? If so, are there any free system monitoring tools for Windows machines? I have the capacity to build a *NIX machine if need be as the main server for this, but didnt want to if I didnt have to. Any suggestions would be helpful.