I'm assuming you are running Windows Vista Home Premium, and these instructions will be for that operating system.
in the Start menu, right-click on 'Computer' and click 'manage'.
under 'Storage' in the sidebar, click on 'Disk Management.' You will see on the bottom most likely Disk 0, it will appear to be in two segments, a Healthy (C:) volume and one that says 'Unallocated'. Right-click on the 'C' partition and click 'Extend Volume.'
the Extend Volume Wizard will launch, and walk you through the process. You should be able to click 'next' throughout the wizard, but read it to make sure. That should give you close to 120 gigs. (after formatting)