I just encountered the error message "Failed to add device Microsoft Synthetic Ethernet Port" or sometimes the VM crashed after installation or changing VM Setting.
I thought it is the Virtual Switch which is giving the problem. So i decided to unmap Virtual Switch, delete existing Virtual Switch and create a new Virtual Switch. However the problem still exist.
After several investigation, manage to locate the problem.
Simple. Just exclude the folder where you put your VHD and VM Configuration files. I'm using Forefront, so below is the snapshot of where you can put the exclusion.
If you're using other type of antivirus, i advise that you to put exclusion to avoid any problem on your Hyper V host.
[Update- 13 June 2010]
OfficeVDI folder is the location where i consolidate VM configuration,VM Images and VM Snapshot.
You should exclude Vmms.exe and Vmwp.exe as well which located in C:\Windows\System32.
Link provided by Michael. For more info, please refer to http://technet.microsoft.com/en-us/library/dd283088.aspx
Link provided by Noel. For more info, please refer to http://support.microsoft.com/kb/961804/en-us
Thank you for sharing.