It just occurred to me that I haven't defragged my hard drive in...maybe ever. I used to do this all the time with my older PCs. Does it not matter anymore?
Fragmentation isn't nearly as big a deal as it used to be (thank goodness). It occurs when blocks of data are scattered out of order across your HDD, which makes it do extra work to read a file, but that happens a lot less that it did ten years ago. I haven't defragged my main drive in ages and it's at 1% fragmentation. No big deal. Still, you should defrag now and then to keep your drive in good shape.
First of all, open up Disk Defragmenter (or Windows 8's 'Optimize Drives'...either way, just type 'defrag' in Windows search) and check to see if it's set to a schedule. You may have been defragmenting your hard drive at 1 am every Wednesday this whole time.
If so: just leave it! Or set it to a time that makes the most sense for you—whenever your PC will be on and not in use—and then go back to what you were doing. If you were making a grilled cheese sandwich like I am, please be aware that it may now be burning.
Wait! Sorry, let it burn for a bit. Do you have an SSD? If so, don't defragment it. Not only do you not need to, but defragging an SSD will shorten its lifespan. SSDs can only take so many program-erase cycles (erasing and writing data, basically) and all defragging will do is run it through a bunch of these. PC World tried out a few defragging programs that offer 'SSD optimization,' but even they weren't much help. So just leave your SSD out of all this.
Sorry if I scared you, because you probably don't need to do anything. Windows 7 won't select SSDs by default, and the Windows 8 optimization tool isn't actually defragging them. And if you haven't been worried about it until now, you almost definitely don't need a third-party tool.