Patio has the right solution, but I understand your frustration. Microsoft.com has a list of things you can do without triggering WGA. I don't have it handy, but changing one piece of hardware is not supposed to trigger it. They probably are protecting themselves against cloned disk images being distributed to different computers, and whoever wrote the routine didn't think about considering whether the original drive was still present, and whether Windows was actually installed on the drive, instead of just detecting the presence of another drive serial number. I know. M$ is getting tight. Just a little too tight. In trying to stop the pirates, they are causing inconvenience and annoyance to the legit customers. But who ya gonna call? Ghostbusters? This kind of thing gives boost to Mac and Linux promoters, but face it, Windows is too established to make a change for most people.
