You need to check if there is any unallocated space on the drive. To do this, you need to use Disk Management. Start -> Run -> type "compmgmt.msc" (no quotes). Then, in the left pane, find Disk Management. Your disks should appear in the bottom pane showing the partitions on the drive. If your drive has unallocated space, you need to create another partition to use that space.
Use this to make a new logical drive with the unallocated space.http://support.microsoft.com/kb/309000