VC++ Directories in VS2010 are project-specific. There are no global settings like in VS2008.
There is INCLUDE environment variable which corresponds to $(IncludePath), so check it first.
Formerly VS saved all global settings in Documents and Settings\\Local Settings\Application Data\Microsoft\VisualStudio\\VCComponents.dat so try to delete such file if it is present for all VS versions. I don't have such file for VS2010, but it's worth checking in your case.