Srs halp thred guise

I wrote a quickie program that does a 2D random walk.  It works fine for a while, but after around 250,000 loops, it stops drawing.  WAT'S WRONG /PROG/?? D:

void DoStuff(HWND hwnd, HDC hdc) {
    WINDOWINFO wnd;
    int xmax, ymax, x, y, r;

    srand((unsigned int)time(0));
    GetWindowInfo(hwnd, &wnd);

    xmax = wnd.rcClient.right - wnd.rcClient.left - 1;
    ymax = wnd.rcClient.bottom - wnd.rcClient.top - 1;

    x = xmax / 2; y = ymax / 2;

    while (1) {
        SetPixel(hdc,x,y,0);

        if ((r = (rand() % 4)) >= 2)
            x = (x + (2*(r % 2)-1) + xmax) % xmax;
        else
            y = (y + (2*(r % 2)-1) + ymax) % ymax;
    }
}

r % 2
:(