The easiest way for you is to recreate the 2nd partition again , make it d-drive and move data to the d-drive. that way you free up room on the C and keep data separate from prorams (on C)
all other options (done properly) would involve backing up all your data, creating (if you vane not done so) new instalation disks, then chnging the disk partitioning (effectively wiping all your data) and then installing the OS on the 1 120GB drive, reinstall all your programs and restoring your data.
I am not in favour (personal preference) to use the in-sity repartitioning programs
hope that helps more than it confuses