Technically yes you need a new OEM license for each machine you build.
BUT if your not keeping the old box, just building a new one, you can install it with your XP disk and when it come up for activation it should go right on through with no issues unless you have done it more than 3-4 times with the same SN. My test box for work has been install so many times its not funny. The auto validation doesn't work for it anymore, so you choose the by phone option and 15-20 min later your done. When it asks you why you are reinstalling it say replaced the hard drive.
"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."