PowerPC G4 Silver Door (HELP WITH OPTIMIZATION!)

I need HELP! I have an old G4 (not that old), it's the one with the silver front door, it's a single processor, 756 of ram, 80 gb hard drive..... and for the LIFE of me I have NO idea what is taking up all my HD space.... I only have about 2.5 gb of hard drive space left (I used Grand Perspective to see how much was taken up and what was left)...... I have NO idea what's using up THAT much hard drive space! I know that one big chunk that I could see was called sparseimage...... what the heck is that??????

I've used monolingual to erase languages..... I went through and I THINK I deleted all unnecessary software (when I say i THINK it's cuz I don't know if I did it properly....).... I don't know what else to do...... how do I optimize my mac and get back some of my hard drive!!!! I don't have a lot of stuff on my computer so I REALLY can't think of what the HECK could be using over 70 gb of hard drive! PLEASE HELP!

What OS are you using? If OS 9 a defrag may be the go.

If OS X it allegedly does not require defragging. Try Repair Permission from the booted startup DVD. No photos or anything like that?

Other than that maybe a format and install.