If you installed OneCare and activated a subscription using a LiveID and provided payment information, you can't create a new subscription using a retail key from a store bought copy and the same LiveID. You can, however, renew an existing subscription at http://billing.microsoft.com with a retail key. You can also create a new subscription with a new LiveID.
Based on your message, you installed the trial via download, but then purchased a retail copy. You then removed OneCare and reinstalled from the retail CD (which you didn't need to do). When trying to subscribe, you login and provide the retail key, but encounter an error. If the error states that the PIN is invalid, then you may be typing the key incorrectly.
I hate the font that they use for the key - some characters are miserable to discern. J and I look all too much alike, as do O and 0.
I recommend that you use a magnifying glass to check the characters closely - I find that I need to do this. If you are still encountering a problem with activation, you have about 2 weeks to get activated, so I'd go ahead and open an email request to support, explaining the problem. See the FAQ and use the Trial user option to start the email request. If you can provide a digital image - scan or photo - of the key to the tech that responds, they can validate the key for you.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If the error indicates that the key has already been used and you have not created a subscription, either exchange your retail copy at the store or contact support for more help.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve
