Problems with Absract Classes

[BossOfTheGame]

I have a two classes one is abstract and another is inherited

// this is part of CGUI.h
class CGUI
{
public:
//other stuff
virtual void Init() = 0;
virtual void Paint() = 0;
};

//this is part of LoadScreen.h

#include "CGUI.h"

class LoadScreen : public CGUI
{
public:
virtual void Init();
virtual void Paint();
}

now in my definition of those classes I get problems

//this is part of LoadScreen.cpp
#include "LoadScreen.h"

void LoadScreen::Paint()
{
//stuff
}

void LoadScreen::Init()
{
//stuff
}

when I do this I get 2 errors

error LNK2001: unresolved external symbol "public virtual void __thiscall CGUI::Paint(void)" (?Paint@CGUI@@UAEXXZ)
error LNK2001: unresolved external symbol "public virtual void __thiscall CGUI::Init(void)" (?Init@CGUI@@UAEXXZ)
fatal error LNK1120: 2 unresolved externals