Unused space or free space is space on an existing partition--OR-- it can mean space left on a drive that isn't formatted or partitioned. If you're looking at an existing partition ( ie. "C" ), any space left over would be unused or free. If you're looking at the whole hard drive you could be talking about extra space that is left from creating your C drive partition. It can be confusing, sorry if I was misleading with using "un-used space" in my previous post. What I meant was space on your hd that wasn't already partitioned or formatted.
As for seeing if you have any "un-used" space on your hd, right click on your C drive, go to properties and it will show you how much space you have used, how much if free, and what the total of your C partition (or drive) is. Compare this with the size of your hd and you'll know how much you have to play with.