Activation: product key from old HD?

I recently built a new PC, and installed a genuine copy of Vista Home
Premium, then activated it. Now I have fitted a new HD (I've still got the
old one) and done a fresh install.

When I try to activate, it says the product key is in use. This is a bit
odd, because I don't recall re-entering the it. However, I've got about 10
used product keys, and I don't know which one it was!

Is there a way to discover the product key from the old HD?