If you want to dual boot with Windows, you need to use Boot Camp assistant to create a special partition for it. In addition to partitioning the drive, Boot Camp Assistant loads special software so that Windows will boot. It won't boot on a normal partition on a Mac because the Mac's EFI (the new equivalent to the BIOS) isn't natively compatible with Windows without it. Boot Camp assistant also downloads the drivers you'll need to make your Mac's hardware work in Windows, and lets you burn them onto a CD or DVD.