I suspect that the credentials for the other user have not refreshed for some reason as the subscription is renewed on the other login.
You can try the following:
- Check your PC date and time.
- Check your subscription status by logging in with your subscription LiveID at http://billing.microsoft.com
- Confirm that this is the same LiveID shown to be the Subscription ID in OneCare - Help/About…
- Open OneCare and click "Check for Updates" to have OneCare check for updated credentials
If your subscription is correctly shown at the Billing site, but incorrect in OneCare, please contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator