Microsoft has a free Security Notification listserv service that you can subscribe to. Subscribers will receive notification of security related issues with Microsoft products.

The goal of this service is to provide accurate information to our customers that aids them in understanding, acting and protecting themselves in a timely manner against malicious security attacks. Microsoft's security team investigates issues reported directly to Microsoft, as well as issues discussed in popular security newsgroups. When we publish a bulletin on an issue, it will contain information on what the issue is, what products it affects (if any), how to protect yourself against the issue, what Microsoft plans to do to fix the problem, and links to other sources of information on the issue.

You should subscribe. Visit http://www.microsoft.com/security/.