The Windows NT Performance Monitor not only helps administrators fine-tune performance, it can also give warning of approaching problems before they are noticeable by the computer user. Performance Monitor can also help you spot the activity of a virus (by spotting performance degradation) or an attempted break–in (by tracking logon attempts). Performance Monitor can be set to send an alert to one or more administrators when certain events occur. The "Performance Monitor" chapter in the Windows NT Server System Guide documents the use of this utility.
For a thorough discussion of performance monitoring in Windows NT, see Optimizing Windows NT, in the Windows NT Resource Kit.