If I reinstall snow leapord while having snow leapord on here

I want to start back at square one. If i reinstall snow leopard onto my computer while my existing snow leapord is still on here will that bring everything back to default? There are some files lost in the hard drive that i do not want on there. So I want a fresh OS. or would I have to format the drive and reinstall snow leapord.

17" macbook pro current generation 10.6.2 snow leapord

Hi GrandDesign
If you really want a clean system, erase or zero out the drive first!
I prefer to zero out if I'm going to erase anyway.
Boot from install disc holding C when you hear the chime, select language, go to utilities>disc utilities in the menu bar, and you're there.
Once you are done exit utilities, and proceed with installation.
Cheers
Kane