Private data member problems

[jrw10293]

I was working on a class for the first time the other day and could not get anything but built in data types to work as private data members. I tried creating a test program with the bare minimum code and still could not get it to work. my error is:

'string' does not name a type

I'm sure there is something simple that I'm missing, but what is it?

here's what i tried:
//---------------------------------------
specification file "Problem.h"
//---------------------------------------
class SimpleClass
{

public:
SimpleClass();
private:
string test; //this line is where i get the mentioned error

};

//-----------------------------------------
implementation file "Problem.cpp"
//-----------------------------------------
#include <cstdlib>
#include <iostream>
#include <string>
#include "Problem.h"

using namespace std;

SimpleClass::SimpleClass()
{
test="bob";
cout<<test;
}

//---------------------------------------
main file "main.cpp"
//---------------------------------------

#include <cstdlib>
#include <iostream>
#include <string>
#include "Problem.h" //compiler highlights this line

using namespace std;

int main()
{
SimpleClass ();

system("PAUSE");
return 0;
}