Before we answer that question, we need to talk about what fragmentation is. Think of the files in your computer as boxes in an attic. You root through the boxes to find something you need, you move boxes around, you bring new boxes in and take old boxes out. Eventually, the boxes get out of order and your attic becomes a dreadful mess! Defragmentation is the process of rearranging all those boxes and freeing up as much floor space as possible.
Not all computers need to be defragmented, and some do it automatically. If your computer is older (more than five years) and it’s starting to slow down, you may want to think about defragmentation.
Before you click the Defragment button, check out this article on defragging and solid state hard drives and this article on defragging Macs. If you decide to defrag, you can find good instructions for Windows 7 here.