Very odd error, please help

[Aragorn]

Hi! it's me again.
I'm in trouble (again): I'm writing a program for simulating a neural network.
I'm working on a graphical interface driven editor for the network. The program displays a window with a picture of the network, and the first thing to do is to be able to drag cells across the window. I use message handling for mouse events and BeginPaint & EndPaint for drawing it. The program compiles fine (nevermind warnings) and runs. Then I select "Network Editor" in the "Window" menu and it opens the editor; fine...
Now I try to drag a cell: the usual way, click on it and move. It goes fine for a while and then it crashes. I get

Thread stopped... : Fault: Access violation at 0xbff51864: write of address 0x40323c

What in earth is happening??????
After this I click OK and go to the CPU window.
I can't find out what's the problem. I may be doing something wrong but i really don't know what.
Here are the parts of my program which are of some interest:

#define SIZE 30			// Diameter of cell

class cell {
public:
   int x,y;
   
   void draw(HDC handle) {
      unsigned int i;
      Ellipse(handle, (this->x)-SIZE/2, (this->y)-SIZE/2, (this->x)+SIZE/2, (this->y)+SIZE/2);
   }
};

class network {
public:
   network(){};
   network(unsigned int in, unsigned int out, unsigned int middle);

   void DrawNetwork(HWND hwnd);
   cell* select_mouse_pos(int x, int y);

   cell* add_cell(std::vector<cell*> in, std::vector<cell*> out);
   cell* add_input(std::vector<cell*> cells);
   cell* add_output(std::vector<cell*> cells);

private:
   std::vector<cell*> incells;			// pointers to input cells in network
   std::vector<cell*> outcells;			// pointers to output cells in network
   std::vector<cell*> maincells;		// contains inner cells of network; those that
						// do some calculation
};

cell* network::select_mouse_pos(int x, int y) {	// Returns pointer to cell pointed by the mouse
   double r;
   int a, b;
   std::vector<cell*>::iterator first=maincells.begin();
   std::vector<cell*>::iterator last=maincells.end();

   while (first!=last) {
   	a=(*first)->get_x();
      b=(*first)->get_y();
      r=sqrt((a-x)*(a-x)+(b-y)*(b-y));
      if (r<SIZE/2) return *first;
      first++;
   }
   return NULL;
}

void network::DrawNetwork(HWND hwnd) {
   HDC hDispl;
   PAINTSTRUCT *lpPaint;
   RECT update;
   unsigned int i;

   if (GetUpdateRect(hwnd, NULL, 0)) {
      BeginPaint(hwnd, lpPaint);
      hDispl = GetDC(hwnd);
      for(i=0; i < incells.size(); i++) {
         (*(incells[i])).draw(hDispl);
      }
      for(i=0; i < maincells.size(); i++) {
         (*(maincells[i])).draw(hDispl);
      }
      for(i=0; i < outcells.size(); i++) {
         (*(outcells[i])).draw(hDispl);
      }
      EndPaint(hwnd, lpPaint);
   }
}

void ManageCellDrag(HWND hwnd, cell *p, int x1, int y1) {
   if (p==NULL) return;
   (*p).set_pos(x1, y1);
   InvalidateRect(hwnd, NULL, 1);
   PostMessage(NULL, WM_PAINT, 0, 0);
}

LRESULT CALLBACK EditProc(
HWND hewnd,
UINT Message,
WPARAM wParam,
LPARAM lParam) {

static network N(6,6,5);  // Here i create the network
static cell *movingCell;

   switch(Message) {
   	case WM_LBUTTONDOWN: {
         int x = LOWORD(lParam);
         int y = HIWORD(lParam);

         SetCapture(hewnd);
         movingCell=N.select_mouse_pos(x, y);
         if (movingCell!=NULL) ManageCellDrag(hewnd, movingCell, x, y);
         return 0;
      };
      case WM_MOUSEMOVE:{
         if (movingCell!=NULL) {
	         int x = LOWORD(lParam);
   	      int y = HIWORD(lParam);
      	   ManageCellDrag(hewnd, movingCell, x, y);
         }
      };
      break;
      case WM_LBUTTONUP: {
      	ReleaseCapture();
         movingCell=NULL;
         return 0;
      };
   	case WM_CLOSE:
		DestroyWindow(hewnd);
      break;
      case WM_DESTROY:
//         PostQuitMessage(0);
      break;
      case WM_CREATE:
      	movingCell=NULL;
      break;
      case WM_PAINT:
      	N.DrawNetwork(hewnd);
      break;
      default:
      return DefWindowProc(hewnd, Message, wParam, lParam);
   }
   return 0;
}

and the loop:

while(GetMessage(&Msg, NULL, 0, 0) > 0)
   {
      TranslateMessage(&Msg);
      DispatchMessage(&Msg);
   }
   return Msg.wParam;
}

I'm running on Borland C++ 5.02.
The attachment is my project: At least i would like to know if it crashes on other computers also, since i think this is a very odd error. please try to run it and drag a cell for a while. If after 10 seconds you didn't get an error then i'm in serious trouble!

TRY TO DRAG ONLY CELLS IN THE MIDDLE COLUMN, THE OTHERS DON'T WORK, YET.

I have some guesses, though most probably i'm wrong on it;
1. movingCell eventually pointing to null and trying to paint (*null).draw(). How did it happen?
2. compiler bug (no way, eh?)

i don't know what to do...