For instance, say your PC is especially slow today; a quick look at your system usage may reveal that your system is running low on memory (physical or virtual).
A full hard disk can be another cause of a slow PC. On another note, high network adapter usage can explain why you cannot connect to certain sites that you normally can access.
Windows has built-in feature for monitoring system resource usage, but it is far away from your hand unless you had created a shortcut to that.
QuickSYS Informer is a handy freeware utility that enables you to immediately see the status of your local system resources.
It's professionally designed interface is very intuitive as it makes use of horizontal bars to illustrate real-time system usage. QuickSYS Informer (download it at source) can track the CPU usage, Hard disk usage, Memory usage, Network adapter use, and Battery life (for notebooks).