Some programs will slow your computer down even when you are not using them. The authors of these applications arrogantly assume that their application will be your whole world. I call these types of programs pigs because they use a lot of your computer’s resources (slowing it down). They continuously use these resources even when you are not interested in using them.
Some examples are, iTunes, Frostwire, Skype, FaceBook, My Space, Live Messenger, AOL. These applications will routinely install themselves such that they start as soon as you start your computer and run continuously. Most people would not choose to do that if they were given a choice. You normally want them when you want them, not all the time. Why slow your computer to a crawl so that one of these is already started instantly when you want it, when the alternative will be to start it and wait a few seconds? If you are careful in the installation process, you can usually prevent this setting, but if you missed it, most of them have Options sections within which you will find that particular setting.
Also, when you are done using these applications, check the System Tray (the area on the bottom right of your screen next to the clock) and see if it left an icon there. If it did, it is likely still running. Right-click (or left if you are using your mouse in left-hand mode) and choose Exit.