While Windows features its own task manager, there's always the possibility to use better tools with extended functionality like Webkeysoft Process Manager.
This piece of software displays a list with all the applications and processes that are currently active in your system. So far, there's nothing different from the Windows standard tool. But Process Manager goes a step beyond and offers extended information about each process, as long as some interesting options like the ability to show the file responsible for each process or a list of dependencies for each active process. This last feature comes in especially handy when you can't delete a file because it's being used by some unknown application.
The program includes some other nice details, such as a couple of meters to show CPU and RAM usage and a startup list where you can see the processes that are run when Windows launches. You can even configure it to replace the Windows standard task manager. But if there's something I would definitely change in Process Manager is its interface. The usual XP look would fit the program much better than that blueish skin.
Process Manager enables you to see what's going on in your computer with a high level of detail.