then look for the administrative tools section and click "Create and format hard disk partitions"
a window will pop up, look for the drive in question and if there is a box with a black header then it has unallocated space click on that and create a partition with that empty space, sometimes windows installs its files in only the area that it needs, usually 16 gb not counting other system files. ive had issues with the disk management utility before where it would not let me allocate space in a drive.
I hope this helped and if you have any questions just ask, ill be looking at this topic often.