Thanks to Saikee for suggesting this method.
First install Windows to it's own hard disk. Then add the new hard disk for Linux making it the master and make the Windows disk the slave. Install Linux to the new hard disk and allow Linux to install it's boot loader (I recommend using GRUB over Lilo) to the MBR of the new Linux disk. Because the Linux boot loader is written to the MBR of the new (master) Linux disk, it will not interfere with the MBR of the original (slave) Windows disk. The Linux boot loader will automatically recognize the presence of the Windows installation on the second hard disk and will add a menu item for booting Windows.
If you want to remove Linux, simply swap the Windows disk back to being the master drive and all will be restored.
If your Windows installation fails to boot giving a file system unreadable message, try remapping the drives by adding the following two highlighted lines to the Windows section of your /boot/grub/grub.conf file:
title Windows XP
map (hd0) (hd1)
map (hd1) (hd0)