I recommend ActivXperts Network Monitor. Runs as a service on Windows 2000 (and Windows NT) machines. The thing I like about the product is its client program. It can be installed on any Windows NT or higher desktop system, and shows real-time monitoring information, and allows you to add/change monitor rules. WMI and ADSI can be used inside VBScript scripts. About 20 check rules are built-in, but VBScript provides you as many chec rules as you want.
I monitored the engine with the Process Viewer of Win2000 resource kit. This thing is really ,multithreaded, with the processor load spread out over my dual processor system.
Native support for SMTP mail (no IIS required), and native pager support (by using Hayes compatible modem).
Very stable product. Download site:
http://www.activxperts.com/activmonitor.