What OS are you running? I'm assuming a version of Windows.
On 7 and below; Click start, then right click on Computer and select Manage. On the left pane, select Disk Management
On 8 up, Press Windows Key + X on the keyboard and then select Disk Management.
If this unallocated space on the same disk as your C drive, just right click on that picture box and select
Extend Volume.
View attachment 7444
Click Next twice and finish and that will extend that drive to the entire available physical drive.
EDIT:
Beat me to it!