Returning hard drive to virgin state (clean out of box)

I am being driven mad by this problem and need some expert help!
Bought a new 320gb hard drive and installed on my macbook pro. after installing the operating system and upgrading to Leopard I had a good 300gb of capacity.
Tried to install Avid media composer and discovered that it is not compatible with Leopard. I then did a clean install of OSX back to 10.5 but I have lost some 60gb of space from the hard disc?
I have tried a zero count etc. on the disc but the volume size has been reduced from a 320gb drive to a 265gb after installing Leapoard and back to Tiger.
Now the big question...How can I get the full volume back on my drive 320gb drive, as it was out of the box. I have tried everything but nothing will return the missing space?
I would really appreciate an easy solution as I feel sure this must have happened to others.
Thanks for looking and I look forward to some genius help.

A complete reinstall of tiger should get all your space back.....weird . If you put in your disc and reformated your drive using disc utility on the disk (not the app on your drive) it should completely wipe your disk clean and get your space. Then install tiger.

Do you use FileVault to encrypt your contents? If so... read this thread:
http://www.mac-forums.com/forums/showthread.php?t=101343&highlight=filevault

Also run Disk Utility and make sure you have one single partition that is the full size of the drive.

I had a similar problem. Run disk utility from the leopard install disk and repair the disk. That should put your HD space back to normal