Reinstall OneCare by going to http://onecare.live.com and activate/subscribe by signing in with the same LiveID you originally subscribed with.
Once you have activated OneCare, make the PC a Hub in your OneCare Circle - open OneCare, click on Your OneCare Circle. You'll see the new and the old PC listed (and any others on your subscription) after signing in to make your PC a hub. CLick "remove this PC" from the details of the old PC name.
If the replacement drive and the old PC are using the same name, you don't need to do any of the above as the reinstall would be seen as the same PC.
-steve