If your PC starts to slow down after you’ve been using it for a while, or one or more programs completely stop responding, you can use the Task Manager, a utility built into Windows, to help regain some of your system resources

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• Task Manager is a tool that allows you to easily monitor your computer’s activity and which helps you control the programs, services and processes that are currently running. You can also view details about the network traffic, the performance of the computer and the number of users that are connected at a given time. In this article I will show you how to use this tool and I'll present you its most important features and options
  How to use the Task Manager | Windows Vista for Beginners.
• One of the nicer upgrades in Windows Vista is the ability to see the file name of a running process through the Task Manager. How many times have you seen a process in Task Manager but couldn’t figure out where on earth it was located? This ability isn’t turned on by default, but it’s simple enough to do.
  Use Windows Vista Task Manager to See File Name of Running Process | the How-To Geek
• The best way to speed up your PC is to give it more available memory—but freeing up as much of the memory you already have is cheaper and easier than cracking open your case to install a new stick of RAM. It takes a lot of memory to get a modern Windows system up and … well, not doing anything in particular, really. Between cryptic system processes, confusing “helper” applications, and background utilities, a whole lot of who-knows-what gobbles up memory and slows down your work. While a lot of programs claim to optimize memory use and pare down processes, Windows’ built-in Task Manager can do the job just fine, given the right tools and know-how. Today you’ll extend and empower Windows’ Task Manager to speed up your system by cutting out unnecessary memory hogs.
  Reclaim Memory by Mastering Windows’ Task Manager | Lifehacker