You paid before installing? Did you have a subscription already that was expired? If so, why did you not renew using the same Subscription LiveID?
Let's assume you have an expired copy of OneCare on your PC activated with LiveID_OLD and you just bought a new subscription with LiveID _NEW. If so, you must uninstall the old copy and install again and activate using LiveID_NEW that you signed in with to buy the subscription.
I'm still not clear how you proceeded to pay for a subscription and are now trying to install while OneCare is already installed.
-steve