Must be a partitioning thing. Start drakdisk ([alt]+[f2] drakdisk), enter your root password, and you should see some partitions. There you can enlarge partitions, and make partitions smaller.
Just a quickie - I'd backup all my important data/settings etc to a USB or DVD
before modifying your partition sizes, as in some cases it may "misbehave" therefore backing up is always a wise thing to do (unless of course your the 1 in a 100 that do daily backups

)
Another thing to take into account is that some Windows systems come with an "hidden" hard disk partition which has the Windows recovery image on it, but I think that's normally only approx 5-10GB in size, not the 100GB your "missing"