Sounds like the 80 GB drive was the boot drive at one point in the history of this machine.
I suggest that you disconnect all drives except the 120 GB drive with Win7 and the DVD drive, boot to the Win7 DVD, and choose a Repair installation. It will look at the Win7 instance on the 120 GB drive, notice that there is no Master Boot Record / boot loader, and install them on the disk. They live outside of partitions, so copying partitions won't solve the problem.
Note: yes, you could use clone software to copy the boot files from the 80 GB drive to the new drive, but it makes more sense to boot off of just one drive. That's why all the others need to be disconnected when you do the repair, to ensure that the 120 GB drive is made bootable.