Will it activate? No.
The issues of licenses for XP are well discussed so let's just say you can use that CD as many times as you feel like it. Microsoft has teeth in the activation so in 28 or so days you'll understand what has to happen next.
Q. Do you have to buy a new CD?
Q. What do I have to buy?
A. Only what you want to.
I think I know what I'm going to hear, but, just for clarity, here goes.
I am building a new computer from a barebones kit. I have an XP Home OEM disk which I am running on my present computer. Can I reuse that disk? Or, do I have to buy a new one? I was prepared to do so, but I thought I would see if it is really necessary.