Solved!
I'm trying to remotely manage a Hyper-V server from my desktop PC. Following these instructions, I've downloaded the Hyper-V Remote Management Update for Windows Vista for x64-based Systems (KB952627). When I attempt to install it, I get the error "This update does not apply to your system."
I'm running Windows Vista SP2, 64-bit.
Edit: I've also tried Update for Windows Vista Service Pack 2 Management Tools for Hyper-V for x64-based Systems (KB970203), to the same results.
Solved! I found this update: http://www.microsoftpost.com/microsoft-download/update-for-windows-vista-for-x64-based-systems-kb952627/ It worked.