Well are you running the Bootcamp partition through Parallels or did you do a whole new install? If you're running the Bootcamp partition, I wouldn't think you'd need to reactivate it. If it's a whole new install...
Well, if you're only trying out Parallels it doesn't really matter if you activate it now...you can still use Windows for thirty days without it. But will you be using your Bootcamp partition or a whole new one if you do end up getting Parallels? If you use your Bootcamp partition, like I said, you shouldn't need to activate it again. If you want a new install, then it's technically illegal, but there are a few ways you can go. The illegal way would be to download a "crack," but I can't discuss that kind of thing here. A semi-legal way would be to use the "Activate by Phone" option when it asks you to install. It will give you a phone # to call Microsoft. Tell Microsoft you updated the **** out of your old PC and had to reinstall Windows, too, but it doesn't recognize it as the same computer anymore because of all your hardware upgrades. They will give you a new activation code.
As a sidenote--also try out Fusion. Personally, I use Fusion and like it. From what I hear, most say it's better than Parallels.