http://support.microsoft.com/kb/954958
BTW, Windows 7 and Server 2008 R2 have Hyper-V HAL drivers in them, so when those two are installed as guest OSes, they take advantage of hardware virtualization even during installation, which makes the installation process go much faster compared to other guest OSes, and at the end of the installation one does not need to install Hyper-V integration services.