Jim is correct that contacting support is the way to go. However, you won't have a lapse in coverage as far as I know. Sign on at
https://billing.microsoft.com with your subscription LiveID to check the key that was used (part of it will be displayed). In addition, if you have a renew link visible, clicking it should get you a free 6 month extension.
I'm still trying to get detailed clarification on the process, but it is my understanding that a subscription that is expiring now and for the next few months, until Microsoft Security Essentials is released, will automatically extended by 6 months. It may not happen until the actual expiration date, however.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator