f/oss network management software?

[mellowtigger]

I work at a non-profit, and we're looking for free (or very cheap) software to do some basic monitoring of our systems.  We use Windows Server 2003, and we have 5 sites.  We'd like something that does basic ping tests, of course, but it would also be handy if it can check available disk space and cpu usage statistics, maybe even the health of the Exchange server.  It should email reports to us (assuming basic email and net services are still available, of course).

Right now we're evaluating a nearly-affordable program, but f/oss would be better since we have a very limited budget.  Yes, we could eventually learn to cobble together our own system from Microsoft utilities, but we don't have the manpower to spare for that project.  So a ready-made program would really help us out.

Any suggestions?