problems with "volumes"/disk space

How big is your drive? Are the two items shown nested, like this:



If so, then the first entry ST96812AS is the physical drive itself, and the second entry, Macintosh HD, is the Mac HD partition inside it. If you have a 60GB drive, it looks correct already. If you were to erase the second entry, then you would erase all your Mac files. It won't let you do that while you're booted off it anyway, though, so don't worry.

Assuming all that is correct, the next step is to look for the files that explain the lost space. The best thing to do is to go to terminal and start looking around using "du -h -a". Try that in the root directory and also in the users directory. Possibly with "-d 2" in your users directory. Otherwise get an app that helps with disk space usage analysis.