All Windows VM - Virtual Disk Service start/stop every minute

Dear all

I recently discovered that all my Windows VM's (2003/2008/2008R2) start and stop the virtual disk service:

System Log File:

08:37:27 Virtual Disk Service --> Service started
08:37:27 Service Control Manager --> The Virtual Disk service entered the running state.
08:37:28 Virtual Disk Service --> Service stopped
08:37:28 Service Control Manager --> The Virtual Disk service entered the stopped state.

and i have this entry every minute.

I removed the DVD Drive from the VM but no change. What could be the reason for this? All my physical servers with the same backup program haven't this behavior.

Thanks for any hint

JBAB