And it shows you how many gigabytes in windows?
What does disk management show you? Is it possible that it has been partitioned smaller then its full size?
Here is an example of a drive not partitioned to use all space. Also note that a 2TB drive becomes 1.81TB because drive makers use decimal numbers for gigabytes and computer are binary. so 60000000 bytes(what a drive makers calls 60gigs and can be divided by 1024 then that number divided by 1024 again to give you a binary value) = 57.220458985375 gigabytes.
http://img51.imageshack.us/img51/9752/partitionv.png