--UPDATE--
I thought it was fixed, however, this was just one error and whilst that got over one hurdle, the bigger install still failed.
After running the update once, it extracts all the individual components to a folder on your hard drive - for me, this was c:\temp\ext47334 - I was able to run every KB on its own, apart from `aspnetwebpages.msi`
This keeps failing with:
"Error 1721. There is a problem with this Windows Installer package. A program required for this install to complete could not be run. Contact your support personnel or package vendor. Action: WebConfigCA_Remove, location: C:\Program Files (x86)\Microsoft ASP.NET\ASP.NET Web Pages\v1.0\WebConfig\WebConfigCA.exe, command: -u "
I have tried numerous solutions - manually launching that command, removing it from add/remove programs (which fails with another error), deleting the folder along with many others - but they all fail.
I have no idea what is wrong here at the moment but will update once it is sorted.
Update: As I thought this is 100% related to the "Microsoft ASP.NET Web Pages" Component.
It is now Solved! Thanks to Matt Garvenn on the ASP.Net Forums, it is down to the Visual Studio SP1 update placing trailing backslashes in two registry keys.
Steps to workaround are:
1. Remove the trailing backslash from the following registry keys:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET\4.0.30319.0\Path
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\ASP.NET\4.0.30319.0\Path
2. Uninstalled the old version of "Microsoft ASP.NET Web Pages".
3. Add the trailing backslash back to those keys.
4. Install MVC 3.
问题找到了就好解决了。使用7-zip解压缩AspNetMVC3ToolsUpdateSetup.exe文件：目录里如下两个文件：
aspnetmvc3.msi和 aspnetmvc3vs2010tools.msi，按照先后顺序安装即可。这样mvc3.0在VS2010里就可以使用了。