Ideally you would back up the configuration and data to tape or desktop and re-install (you might need to deal with any "non standard" MySQL data specifically).
However with 160gb you will be unable to do this.
I suggest you install a new HD, install to that - make sure the "old" drive is not connected while you do it. Then re-connect the old drive, re-boot, and mount the drive and then copy the data over.