If your computer is running slowly or acting as if it does not have enough memory installed, you can use software that comes with the operating system to see what's going on under the hood.
On a Windows system, open the Task Manager program by right-clicking on the Windows task bar and choosing Start Task Manager. (You can also use the keyboard shortcuts Control-Alt-Delete or Control-Shift-Escape.) In the Task Manager window, you can see all of the programs, services and processes currently running on the computer. You can also use it to close an application that is not responding; Microsoft has a demonstration video at bit.ly/VSBUmk.
On a Mac, open the Activity Monitor program by going to the Applications folder, then to the Utilities folder, and double-clicking on the Activity Monitor icon. On Macs with the Launchpad feature, just click the Launchpad icon in the Dock, click the Other icon and open Activity Monitor. Here, you can see the amount of memory being used, what programs and processes are running and other system information. Apple has more information about the Activity Monitor at support.apple.com/kb/HT1342 and steps for quitting a frozen or memory-hogging program at support.apple.com/kb/PH5147.