If this is a new install, check your PC date and time. I suspect that it is incorrect. Once you have corrected it, reboot the PC, open OneCare and follow Jim's instructions to activate.
If you had a previous subscription to OneCare and you are trying to renew with the retail key following a reinstall, but you used the same LiveID as the expired subscription, you will need to renew the subscription at http://billing.microsoft.com and use the retail key/PIN to renew. It can take up to 24 hours for the updated subscription status t appear in OneCare.
-steve