No. But you might have to reactivate it. Depecnds on other changes made and when they were made. If no hardware changes were made in the last 6 months, you might not have to reactivate. On the other hand, if many things are integrated on the board you might. It is hardly a hassle, if it has been more than 6 months you should be able to reactivate online. Otherwise, it is a simple call to MS and 5 minutes of your time.
If you are going to reinstall Windows (which I recommend, especially if going to a new and different chipset (VIA to nForce2 for example) you will have to reactivate. Reinstalling will eliminate any portential driver conflicts and will better streamline your new install. You could do a repair of the current OS, but I prefer to reinstall and get a fresh start with my fresh hardware.
\Dan