Tried to add this as a comment to Rob Prouse's posting, but the lack of formatting made it unintelligible.
In Visual Studio 2010, the "Tools | Options | Projects and Solutions | VC++ Directories" dialog reports that "VC++ Directories editing in Tools > Options has been deprecated", proposing that you use the rather counter-intuitive Property Manager.
If you really, really want to update the default $(IncludePath), you have to hack the appropriate entry in one of the XML files:
\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Platforms\Win32\PlatformToolsets\v100\Microsoft.Cpp.Win32.v100.props
or
\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Platforms\x64\PlatformToolsets\v100\Microsoft.Cpp.X64.v100.props
(Probably not Microsoft-recommended.)