I can't erase my HDD on My Macbook

Ok I'm trying to erase my hard drive so I can install snow leopard. I tried to install it and it said something about my disk being messed up. So now I'm using my macbook with an external HDD that I cloned last night. But I'm tring to erase me internal HDD.

Here's what it says: Disk could not be unmounted.

So how do I unmount it?

Oh okay I figured it out, I had a program opened from my internal HDD so I couldn't unmount it haha

Sorry guys/girls!

try starting up off the original restore disks

when you get to the restore / install screen, select Disk utility from the utilities menu at the top and see if you can reformat the drive from there

Check the link in my sig to do a clean install of SL OS 10.6