Incidentally, the answer to this post is:

Microsoft allow about 4 reinstallations of Windows on an OEM license by automatic internet activation. Furthermore, if the machine checksum changes significantly, due to core hardware changes, automatic activation will fail.

This is to prevent a product key from being used on multiple machines.

A typical way to circumnavigate this is to use telephone activation and simply write down the authentication code. This works as long as the machine checksum doesn't change. Another method is to use telephone activation and simply hang up before confirming the last digits of the authentication code; this apparently does not count toward the total activation count. I never actually confirmed this....it may be erroneous.