Obtaining 64 Bit DVD?

I have recently bought Vista Home Premium and so far have had no problems with the 32 bit edition. However, my system has 8GB RAM and so really I would rather have the 64 bit edition for obvious reasons. However, I have had no luck at all obtaining the DVD. I use language English (Intl) and ship to UK. Then I enter my product key and the captcha. After a brief delay, I always get the following message "We're sorry, the following error(s) have occurred: The Product Key you have entered is invalid. Please try again." Does anyone know why Microsoft will not allow me the 64 bit software?