Find the "restore" discs that came with it.
Put them in the drive. Reboot and hold down the 'c' key to boot off the CD.
Now, on the CD there will be an application called "Drive Setup" in the Utilities folder which you can use to format the HD; or, you can highlight the HD icon on the desktop and go to Special -> Erase Disk (menu items).
You're from the PC world... fixing a Mac doesn't revolove around reformatting.
If you like, we can fix the system for you; I suggest you just double click that "restore" icon.
Make sure to back everything up.