After rebooting start pressing the escape key, if this one doesn't take you to the menu you need reboot and do the same with the F1 key and so forth to the F12 key. One of them will probably let you in to the boot menu.
you can install xp to a home built cpu. when activation comes, and you have to call microsoft, just say you are "reformatting." i have done this with another cpu hanging around, and have no problems.
