System Monitor (sysmon.exe, though not to be confused with a newer Microsoft product) was a system monitoring program in Windows 95, 98 and ME that was used to monitor various activities on a computer such as CPU usage or memory usage. The much enhanced equivalent of System Monitor in the Windows NT family, perfmon.msc or perfmon.exe, has been variously called both System Monitor (Windows 2000, Windows XP) and Performance Monitor (Windows NT 3.x-4.0, and Windows Vista and later); its core functionality was introduced as "Performance Monitor" in Windows NT 3.1. (The Windows 9x System Monitor binary cannot run on Windows XP.) This type of application may be used to determine the cause of problems on a local or remote computer by measuring the performance of hardware, software services, and applications.
In Windows 9x, System Monitor was not installed automatically during Windows setup, but could be installed manually using the Add/Remove Programs applet, located in the Control Panel. Performance Monitor is installed automatically during Windows setup, and is available by running perfmon.exe. The Windows 9x System Monitor had few counters available and offered little in the way of customization. In contrast, the Windows NT perfmon had over 350 counters available. Performance Monitor can display information as a graph, a bar chart, or numeric values and can update information using a range of time intervals. The categories of information that you can monitor depend on which networking services are installed on your system, but they always include File System, Kernel, and Memory Manager. Other possible categories include Microsoft Network Client, Microsoft Network Server, and protocol categories.
The functionality of the perfmon.exe program, which was called "Performance Monitor" up to NT 4.0, was migrated to a Microsoft Management Console (MMC) plug-in in Windows 2000 (perfmon.msc), and this plug-in titled itself "System Monitor" inside the MMC, while the "Administrative Tools" shortcut to it was simply called "Performance", The "System Monitor" naming for perfmon.msc was kept in Windows XP. Some third-party publications referred to perfmon.msc as "Performance Monitor" however, even in Windows 2000 or XP contexts.
The name displayed inside the MMC strictly for the original perfmon functionality was changed back to "Performance Monitor" in Windows Vista, although it was also bundled with a Reliability Monitor and with a new performance summary feature called Resource Overview; the whole was called Performance and Reliability Monitor. In Windows 7, the resource overview feature was split to a stand-alone Resource Monitor application, with the landing page for the Performance Monitor in Windows 7 containing a pointer to the (new) Resource Monitor; Windows 7 also moved the Reliability Monitor to the Action Center. A new feature added to the Performance Monitor in Windows Vista is Data Collector Set, which allows sets of accounting parameters to be easily manipulated as a group.
A number of command line analogs for perfmon are also available. The Windows 2000 Resource Kit provided tracelog, while Windows XP introduced the much improved logman.
In 2014, Microsoft released a new tool called Sysmon that is unrelated to System Monitor.