Hyper-V Program Manager
When you install Virtual Machine Additions we make a number of changes to help improve the general experience of using a virtual machine. One of these changes is to enable 'mouse integration' - which is the feature where by you can seamlessly move the mouse cursor in and out of the virtual machine without needing to capture and uncapture it. In all releases prior to Virtual Server 2005 Service Pack 1 - this functionality was provided through the use of a modified i8024 driver (this is the driver used by Windows to manage the PS/2 controller on the motherboard). With Virtual Server 2005 Service Pack 1 we use a mouse filter driver to enable integration instead.
Why should you care? (I hear you say).
The importance of this change is because using a modified i8024 driver caused a lot of interesting problems. The most notable of which was that if you had Virtual Machine Additions installed and needed to repair your Windows installation using the original install CD you would lose the ability to use your mouse and keyboard inside the virtual machine (which is quite a problem). With the new mouse filter driver, repairing an installation works flawlessly.