mike,
like the others, i find this quite perplexing.
based on what you have told and shown us about how you use this computer, my guess is it is more likely that you have filled closer to the 75GB than the 25GB showing. meaning, i imagine the problem is more likely that for some reason our tests are only showing 25GB, and that is in error, than that you have really only used 25GB and somehow 50GB are missing. it is easier to explain this guess (partitions, user accounts, etc) than missing memory (hardware or massive software malfunction).
just to make sure we are all on the same page, would you mind posting screen shots of the following:
run activity monitor (in applicatons>utilities), select the disk usage tab.
finder (showing all disks mounted)
about this mac > more info > ATA > and then the volume with the most information.
cheers