Surprisingly this is not true. A Computer can actually technically run as fast as it did when you first opened the box, as long as it is treated and maintained in the correct way. Usually a computer gets slower as you continually install programs on your computer that have services that are run on Start Up. Your Computer, (like most people I know), becomes slow the more things it is asked to do at once. Also newer programs tend to be more performance hungry as they generally have more features and assume a more modern computer.
It helps to know everything your computer is doing. If your computer is slow, right click the ‘Start Bar’ and select ‘Task Manager’. This should give a list of all the processes and programs that are eating up all your systems power.