Client Hyper-V was one of the things I was most looking forward to when I upgraded to Windows 8. I immediately added the Hyper-V feature and went to installing Server 2008 R2 in the first virtual machine. That went well enough and nothing extraordinary happened until I tried to run Windows Update in the new VM. Try as I might, Windows Update would always fail with the error code 80072EFE. Sometimes it chose to fail with 80072EE2, though.
As documented in a support article, those error codes stand for ERROR_INTERNET_CONNECTION_ABORTED and ERROR_INTERNET_TIMEOUT, respectively. Not too helpful. After searching around for some time I finally found an article that pointed me in the right direction.
As it turned out, Windows Update manages to disrupt the internet connection (not only of the guest, but also of the host):
The reason for that nasty behavior seems to be a bug in the implementation of IP/TCP/UDP offload: after disabling these four settings on the synthetic guest NIC Windows Update worked like a charm: