How do I use Nagios to monitor Windows clients? Are there any alternative solutions available?
Install the nsclient++ agent on windows, then configure your RHEL5 nagios config file accordingly.
There may be other windows nagios agents, or you can configure nagios to use remote probes with SNMP, but I've used nsclient++ and it works well.
Disclaimer, I'm the Zenoss Community Manager.
Zenoss Core will monitoring your Windows boxes, as well as their applications and databases and most everything else on your network. For Windows there is SNMP and WMI monitoring available without requiring an agent installed on the box.
OpenNMS will monitor snything that puts back SNMP (to the best of my knowledge).