TiVo finally launched its High Definition-capable unit. Designed by TiVo for DirecTV, it sports a 250 GB Hard drive and four tuners for $999. But neat new products aside, TiVo faces some daunting challenges. It has not made inroads with cable companies, many of whom have commissioned cheap TiVo knock-offs from companies like Scientific Atlanta. Many owner of these off-brand DVRs don’t even know that they aren’t TiVos, and, having never used the real thing, don’t know the difference.