While good old Task Manager and the new Resource Monitor are great utilities for monitoring system performance in real time, for any more extensive monitoring needs I tend to prefer Performance Monitor. Perfmon, as it is often nicknamed, is an excellent tool that can be used for collecting specific data over a predefined period of time.
We have all had cases where a report comes across our desk that a certain server is misbehaving or running slowly. By the time we get logged in, everything looks normal. Other than Event Viewer, we don't have a whole lot of options for investigating what was happening during the time of the problem. But it might happen again, and if we plan ahead ...