Option 1: Can You get to the hard disk, and take a picture of the label on it? (Hint: turn off the computer and open the case to see the hard disk.) Then we could see what the physical capacity of that hard disk is supposed to be.
Option 2: Can You go to Your Control Panel -> Device Manager and find the Hard Disk? Mine shows as a "HGST HTS721010A9E630 ATA Device." A Google search for HTS721010A9E630 shows it's a 1TB hard disk.
Option 3: Can You go to Your Control Panel -> Administrative Tools -> Computer Management -> Storage -> right click on Disk Management (Local) -> and click on Your C: drive. This will show the partition table. What do You see there? Mine shows "System Reserved 356MB" and "C: 931.16GB NTFS."
Obviously, my machine is not the one with the problem. Mine is a laptop with a HGST TravelStar 1TB 2.5" spinning hard disk in it.
--Andy