Simple make sure that those partitions are suitable for windows 7, NTSF... blah.. blah ..blah. Label the partition you want to install something like "windows7". During the install make sure you choose the correct partition.
Note the size of your partitions....when the install happens, it should recognize the one w/ Windows on it. Check to make sure size matches and then install to that partition. Though I would not be installing a new OS on an ancient 80 GB HD. I'd add a new HD for like $50