Due to increasing exploitation of the Java engine by bad guys, Oracle has strengthened the default security as of Java 7 Update 21, which is due for release today. As a result, you should update to Java 7 Update 21 as soon as possible, to protect yourself from dangerous Java applets. The slight down-side of this change is that it means you will get more prompts. Always read those security prompts and if you don't know what it is, don't allow it to run.
More information from Microsoft Malware Protection Center. Thanks to Susan Bradley for the heads-up.
