Hyper-V Program Manager
Amongst the many varied settings for a virtual machine under Virtual PC there is a 'Mouse' setting which most users do not use or understand. The first confusing thing about this setting panel is that it is only enabled when the virtual machine is running, and has Virtual Machine Additions installed. At all other times this panel is disabled. The second confusing thing about this panel - is 'Why would you want to disable this option anyway?' Most people install Virtual Machine Additions specifically so that they can have an integrated mouse (and other features). The answer that I usually give people who ask me this question is:
'The cannon scene in Monkey Island III'
And after a moment of stunned silence I have to explain:
Monkey Island III has a scene where you control a ships cannon with you mouse and attempt to blow up a bunch of ghost pirates. The problem is that this scene expects to have complete control of the mouse and breaks if integration is enabled. Another reason why you would want to disable this option is for any game with 'edge scrolling' - like Command and Conquer - as with mouse integration enabled it is very hard to actually hit the edge.
Now for those of you thinking that this might be a gaming only problem - I have actually seen this problem with other programs that require complete control of the mouse - like 'Poser'. It's just that I hit this problem most when I am trying to game.