Highlight the C drive (or any other hard drive partition you have), right click on it and select Properties.
From the Property window, select the Tool Taband click on Defragment Now.
You will get a window like that.
With your C; drive highlighted (by default) click on Defragment
You will get the status of your file structure and how it is restructured. It may take a long time (1 hour or more).
You can defragment 2 or 3 times in a row until you get a good quantity of blue continuous segments.
Note that your system may hang (you will have to hard reboot). There is nothing to do about it. But on the bright side, it wont harm your system to try another time. Some times your system needs a couple of tries!