When you say 100GB, do you mean the manufacturer's spec (=100,000,000,000 Bytes) or what the computer would measure (=100x1024x1024x1024 Bytes)?
Same question goes for the 76GB you say you have now.
I presume you're probably reading the initial size from the manufacturers specs. This equals 93GB as measured by the computer, and I guess you're measuring 76GB from the computer. So you have 17GB to find.
/Library will take close to 8GB, /System 2GB, /Applications maybe 3GB, 2GB for /usr and /bin together. That's 15GB. Plus a few more 100s MB for other directories.
By the way, OS X itself doesn't take up that much - the applications and application support files take up the bulk of the disk space.