If you run .NET in Medium Trust and receive an error like this when installing MVC 3:
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
Edit the .NET 4 root web.config (for both 32-bit and 64-bit) and temporarily toggle this line to "true":
Set it back to "false" when MVC finishes installing.