A network that measures downtime in millions of dollars per minute (or per second!) needs a serious, enterprise-level network management tool. Nothing less will do.
The ideal network management platform accurately discovers devices, computers and applications on the network, works on networks of any size and uses computing resources frugally (after all, it performs no data processing - it's there to watch over the network).
It can work within the framework of a global directory (LDAP, for example). It graphically depicts the entire network, subsets of the network and individual devices. It monitors the status and health of every device or computer on the network. It can glean its data from a variety of sources, including agents, probes, SNMP-enabled devices, log files and Windows performance files.