You will need to check all the directories on the drive. But even still you'll notice the numbers will never add up.
Here's why: Your hard drive is divided up into thousands of sections by the file system. Some of these sections are used to store the locations and file names of your actual data. Also if you have many small files, then even though they may only take up a small portion of one of these sections, the rest of the section becomes unusable to any other file. But these two variations should only account for a very small percentage of lost space (probably 5% max).
What you should do is right click on where it says 'My Disk ( C: )' and select properties. There you should see a full brake down of how much space is being used.
You'll also notice that even though your Hard Drive is a 250GB, your computer says its 255GB. The definition of a GB changes depending on what you're looking at. Why they never went to a single standardized version is beyond me.
