To be able to compile code with Visual Studio 2003, you should already have Visual Studio 2003 (IDE and compiler) ...
If you have it, just use VS2003 IDE (with its compiler) for your C++ code and keep using VS2010 for your C# code.
You probably have to rebuild the project from scratch since the project files (dsw, dsp, ... ) are not backward compatible.
Other than that, there are a few google hits that might help you; and I found this here ,on CP that could help you :
Using Visual Studio 2008 IDE with Visual C++ 2010 compiler