Well, the ORIGINAL :-) Windows XP CD should be bootable. If it's not or if for some other reason you can't boot from CD, you'll have to create the boot floppies for your version of XP. Go to
and download the boot disk creator for your specific version of Windows XP (in your case XP Professional, but note that there's a difference between SP1 and the original one). Create the boot floppies (six of them) and boot your Virtual Machine with them. This should get the setup process going.