System performance can slow down over time for many reasons. Adding new hardware, or misconfigured hardware, can tax your machine beyond it's original specs and cause it to act sluggishly and even freeze up.

Viruses and Maleware from the internet can sap your system's precious resources to the point where it can take several minutes to perform even the simplest tasks.

Computers are very suseptible to damage caused by overheating. This could occur if the venting ports on the computer case become blocked with dust, lint or pet hair, or the computer is placed in an area with poor ventialtion.

Another reason for reduced performance is insufficient Memory. When your machine was new, it had all the Memory it needed to run the Operating System and the installed programs. As you add new software, the Operating System grows and so do it's demands on Memory.

Fragmented and cluttered Hard Drives can also cause perfomance issues. Over time, you save and delete many thousands of files to and from your Hard Drive which can cause it to become fragmented. The more fragmented your drive becomes, the less efficient it will be at storing and retreiving files.