You may experience one of the following issues on a Windows Server 2008 system with the Hyper-V role installed or Microsoft Hyper-V Server 2008: When you attempt to create or start a virtual machine, what is happening is that users are having problems starting virtual machines after they install antivirus software in the management operating system.  The root cause of the problem is that a number of these programs monitor file access in a way that interferes with Hyper-V’s attempts to open virtual machine files.

More info: KB961804