Restart your PC in ms-dos mode. At the command prompt, type fdisk and press enter.
If a message appears about large disk support press "Y" and enter if necessary.
If you have more than one hard drive select the option to change drive, usually option 5, and then select view partition information, option4.
It may be that your drive has been partitioned for some reason and only half the disk is being used, fdisk will confirm this and allow you to create a partition to use the remaining space.
Be careful how you use fdisk, as if you accidently delete a partition, then it is gone for good.