What do I do with Windows after making a new computer - with an old hard drive!

I'm going to be building a computer, and I've got all the parts I need.

But, I left out the hard drive because I have an old one with XP on lying around.

Now, once I've built the computer, and I put in the old hard drive what will happen?

Obviously Window's is not going to like it, because it's a different chipset, and a whole new motherboard!

But, how do I "repair" it so it boots (I have an XP CD)