nidge20 user said 1 hard drive and not 1 PC. Don is correct, you can only install 1 key on one PC. However, you can have a back-up drive for your PC and have the same key installed on that hard drive. While that is legal, Windows may call one of the installations as not genuine, but I doubt it. If I uninsulated a 32bit version and replaced it with a 64bit version, or vice versa, which I have done, I have never had a problem with the key. And if I were to pop the 32bit drive back in I believe it would update also. So I believe you can have both versions installed on two different hard drives, as long as those drives go into the same computer. .
Two PCs or two partitions, it doesn't matter. You can only activate one install at a time per activation key. The activation key is proof of license ownership which authorizes the use of only one Windows install. If you want another install of Windows (bit version is irrelevant), it doesn't matter if its on a different PC or partition, you will still need another activation key.