Macbook Pro

I have 500gb capacity of HDD
I have only 150gb used files that I can see
But I only have 166gb free space
I can't see 200gb files used my HDD

Anyone knows how can I find that files and delete it without formatting my HDD?

Grand Perspective (free)

Cheers!

Thanks for reply i just upgraded my mac into Lion OS but i check all the file on my HDD it has only 150 used file and i have files in my HDD that is invisible

Before that i just deleted my original user in my snow leopard account and i don't know if the file are all deleted also.