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.
Yet another vote for NSClient++. One of the things I really like about it is the ease with which you can add whatever checks you want through custom scripts. A number of scripts are included, which makes it easy to see how things are done.
IMHO, the best option is SNMP. SNMP has some major advantages: it's cross-platform and a an industry-standard. On the other hand, you can also use the SNMP agent to collect data and generate graphs.
Another vote for the NSClient++. I've got it on over 60 Windows servers and excluding a few memory leaks on older win2k machines it has been extremely stable, incredibly easy to install and quick to configure at the nagios end.
OpenNMS will monitor snything that puts back SNMP (to the best of my knowledge).