In Windows 2000 and Windows XP. Performance Monitor (Perfmon.exe) was the expert's tool of choice. It used to be the only way to view real-time details for details about system performance. It's still around in Windows 7, although it's only truly needed for extremely advanced troubleshooting. Here, I'm configuring it to track Committed Bytes and Available Bytes.
For more details, see Windows 7 memory usage: What’s the best way to measure?