If you have used Keenan's advice and switched the RF power out switch to ON, switch it OFF again as this sends 9V current along the cable and is designed for the Sky TV link only. It will not improve the picture quality but could cause problems with your TV card.
As per [email protected] it could be the RF channel that your Digi box is set at, but B4 you change the Digi box settings I would suggest you check the cable as this can cause problems if it has not been made correctly.
If you take a cable direct from the Arial to your TV card. If the picture is clear this will show the problem is not with the cable.
If you then connect your Digi box to your TV card using the RF 1 out put this is usually connected to your video or TV. If the picture quality is OK it could be the RF 2 connection on your Digi box that is at fault. If the picture quality is still poor the RF setting of your Digi box is usually the problem and it will need to be changed. You will need patients as you will need to find is setting not being used by a TV channel or your video recorder you will also need to tune Sky into your Video again.