Trying to give something back to the Community...
If you have installed Windows Vista Service Pack 2 on the system that you are using to manage Hyper-V – you should grab this update:
Note that this is an “update” which means that you need to have the original version of the Hyper-V Management Tools installed first (you can get them from here: http://support.microsoft.com/kb/952627/).
For those of you who are curious as to why you have to do this as a separate download than the whole Windows Vista Service Pack 2 process – the reason is that as Hyper-V RTM’d after Windows Server 2008, we had to release the Vista management tools as an optional separate component on Windows Vista. The result is that we also have to service it separately – as we cannot assume that all users have it installed prior to installing Windows Vista Service Pack 2.
Read the complete post at http://wmug.co.uk/blogs/cliffs_blog/archive/2009/07/08/hyper-v-hyper-v-management-tools-update-for-windows-vista-service-pack-2-virtual-pc-guy.aspx