I'm pretty sure Arch will not help you much with setting up dual boot. It can be done, naturally, it's just you'll have to do it yourself.
First, use Partition Magic only to shrink Windows partition. Then make Linux partitions with Linux tools, during the installation.
During installation of Arch, be sure not to select the automatic option, since it will erase your whole drive. You'll need to go into the manual mode, and make partions in that empty space you've made in Partition Magic.
Finally, you'll need to configure the bootloader, GRUB or LILO, to recognise both operating systems...
... and that's why I don't double boot