EDIT 8-2-2013: A new update KB 2744129 has superseded the original KB 2526776
Please note that this new update now applies to Hyper-V 2008 and 2008 R2 — the original was only for R2.
If you are testing out Windows 8 as a VM which is hosted on a Windows 2008 or 2008 R2 Hyper-V server, then you will need to install the following update.
The issue occurs because the Hypervisor does not handle the one-shot synthetic timer that is also known as an aperiodic timer correctly.
Note that this will require a reboot of the host machine, and if you do not install the patch the host may produce a stop screen, and restart which will impact other running VMs on the host.