I have been battling this for some time. I have tried the fix-it tool without any luck.
I use norton Internet security 2010 and Spybot SD. I tried to install with them shut down to no effect. Also uninstalled both, also to no luck.
I have Vista Home Premium SP2
The update causing issues is KB968930, Windows Powershell 2.0 and WinRM2.0 for Windows Vista