Here you go:

Code:
void openFileDialog (char* filename, char* title, char* filter)
{
     OPENFILENAME o;
     zero(o);

     strcpy(filename, "");

     // Stelle den Dialog ein
     o.lStructSize   = sizeof(OPENFILENAME);
     o.Flags         = OFN_PATHMUSTEXIST | OFN_HIDEREADONLY | OFN_NOCHANGEDIR;
     o.lpstrFilter   = filter;
     o.lpstrTitle    = title;
     o.lpstrFile     = filename;
     o.nMaxFile      = 255;

     GetOpenFileName(&o);
}

// Usage:

void example ()
{
     char file[512];
     
     openFileDialog(file, "open file", "text files (*.txt)\0*.txt\0\0");
     wait_for(openFileDialog);
     
     printf("the file %s has been selected", file);
}