You can find solutions to common issues that people run into with Hyper-V at: http://social.technet.microsoft.com/wiki/contents/articles/hyper-v-gotchas.aspx.  

If your virtual machines are not starting with one of the following errors:
  • The requested operation cannot be performed on a file with a user-mapped section open. ( 0x800704C8 )
  • Failed to Power On with Error 'The specified network resource or device is no longer available.' (0x80070037)
  • The I/O operation has been aborted because of either a thread exit or an application request. (0x800703E3)

 

The likely cause is antivirus software that is installed in the parent partition and the real-time scanning component is configured to monitor the Hyper-V virtual machine files.

To resolve this issue perform the steps documented in Microsoft Knowledge Base article KB961804, Virtual machines are missing in the Hyper-V Manager Console or when you create or start a virtual machine, you receive one of the following error codes: "0x800704C8", "0x80070037" or "0x800703E3".

If the only thing you are running on the virtualization server is virtual machines. why do you need to run anti-virus or other software that you normally run on a Windows Server 2008 R2 server?

Review this short video showing what happens when you enable the Hyper-V role (from Microsoft Showcase), and change your software installation so that all the VMs get the correct software installed as required by your IT policy. Review the best practice recommendations at Planning for Hyper-V Security (TechNet Library) and evaluate if they fit your environment. If your IT policy requires AV in the management operations system (on the host - as opposed to on each VM), see the list of Hyper-V AV exclusions in the primary AV exclusion KB for Enterprise customers (KB822158).