If you're constructing an argument list for an external executable, for instance, all you have to do is quote the path:
string path = @"C:\TestFolder\Input\Some File.rar";
ProcessStartInfo psi = new ProcessStartInfo();
psi.FileName = "program.exe";
psi.Arguments = "\"" + path + "\"";
..which will result in this commandline:
program.exe "C:\TestFolder\Input\Some File.rar"