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NmapWin isn't that bad.

It does actually work. Sometimes it seems to pause for a long time for no reason, but I think that's Windows' resolver library, not really nmap's fault.

It will be...

There is a Windows version of nmap, it will work fine. You can ping scan your network (which just pings the boxes), and easily tell which ones are up.

I'd still recommend a physical audit...

As long as all your hosts are set up to respond to pings, you could just write a script to ping every IP. The ones that don't respond are available.

If you run a DHCP server, then it will know...