(Re) Activating Vista

Hi all,

I bought Vista a few months ago, and all was fine until my motherboard gave
out. I replaced the motherboard and processor, but kept the old hard disk
drive with Vista already installed.

It all runs fine, but Vista is now asking me to activate again, and when I
try the product key that came with the software, it tells me that this code
is already in use.

Does anyone know how I can solve this problem?

Thanks,
Matt