I can be of no help with boot prob. but can tell you that there is NO limit to the number of times you can activate XP though if you have a lot of hardware changes you may get a message on screen to say that you have to contact microsoft for a new activation code. That is no prob. they just give you one over the phone.
You are only allowed one XP on one PC at a time.
EULA (End Users Licence Agreement)