You need to activate(link for subscribe) with your subscription LiveID following the reinstall.
When you reinstall, you will get 15 days before it needs to be hooked up with your account.
If your subscription to OneCare shows expired in OneCare
- 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...
If your subscription is correctly shown at the Billing site, but incorrect in OneCare and your PC date and time are correct, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve