By default, the free-as-in-beer Microsoft Hyper-V Server 2008 is configured for manual updates. In other words, it is NOT configured out of the box to go fetch automatic updates. This is desired behavior for most installs. However, if there are critical issues, like those you might find on the secbuls page, then you must remember to go manually update your Hyper-V Server.
Let's go do that right now.
No, really, stop what you are doing and go patch your Microsoft Hyper-V Server 2008 boxes right now. Here's how:
Note: This may take some time.
You may be prompted with “A restart is required to complete Windows Updates. Restart now?” Click Yes to initiate a restart. By default, running virtual machines will automatically be saved prior to restarting the Hyper-V Server.
For more info see http://www.microsoft.com/downloads/details.aspx?FamilyID=2a429ea3-e33b-414f-ae49-2eead9f5b351