You don't get a product code for OneCare when activated online - only if you pick up a retail box. And the product code can only be used once to create the initial subscription during the first activation. Subsequent to that, all activations for additional PCs or reinstalls are done using the LiveID which is for your subscription.
Where are you trying to log in? Are you failing to activate and getting an invalid password error during activation? Can you log in at http://account.live.com with the ID/Password?
Are you typing in the password during activation or copying/pasting?
Is the password longer than 8 characters? If so, change it to a shorter password of 8 characters or less and try again.
-steve