Quicktime 7 Pro

I bought a software registration key for Quicktime 7 Pro in Sept 2006.

It was working fine until my computer had to be rebooted with the recovery discs and wiped it off it's hard drive.

While still having the original receipt from Apple Store and the original key it won't work with the new version of Quicktime.

How do I get the Pro version back again on my computer using the original key?

Many Thanks

Dave

Compaq Presario Windows XP