Microsoft Knowledge Base Article 300237 contains the following summary:
This step-by-step article describes how to configure your Windows 2000-based server to inform you when your computer resources are running low. Windows 2000 defines the performance data it collects in terms of objects, counters, and instances. A performance object is any resource, program, or service that can be measured. You can use System Monitor and performance logs and alerts to select performance objects, counters, and instances to collect and display data about the performance of system components or installed software.
You can set an alert on a counter so that a message is sent, a program starts, or a log starts when the selected counter's value exceeds or falls below a specified setting.