Like the title says, I purchased Live OneCare this morning via the Hotmail "OneCare" link. The system processed my credit card payment but I did not get to download it.
Sign in with the LiveID you used to purchase with at http://billing.microsoft.com to verify the active subscription.
Assuming it shows as active, download and install the trial from http://onecare.live.com and then "subscribe" when installation is complete, by signing in with that same LiveID to activate the installed copy.