If your Windows installer has an unused product key then you can install it on your Mac.
and, If it is an authentic Microsoft CD, without a manufacturer's logo, such as HP, or DELL
and, If it is a full-install of Windows XP, not an upgrade, with at least SP2 all on one disk
You will likely be stopped by the first step. The installer comes with the new PC, if it becomes necessary to reinstall Windows on the same hardware. The product key will normally be on a label visible on the case. You won't be able to use that same product key on completely different hardware, as you are already using that product key.