After installing win8.1, the hard disk capacity is very small. What happened?

There is such a problem. The reason is that your friend did not format the C drive before installing the system for you, but installed the system directly. As a result of two system files, the capacity of the C drive will naturally become smaller. If there is a Windows (old) folder in your C drive, it is recommended that you do not delete it directly. Please follow the steps below:
First, first enter the computer (this computer), then use the mouse to click on the system disk (usually C drive), then select “properties & rdquo;,

second , choose us to click "Disk Cleanup",

Third, then click "Clean System Files",

Fourth, and finally check "Previous" Windows installation & rdquo; file, and then click OK at the bottom, then wait for the cleanup to complete, and finally delete the Windows (old) folder in the C drive.