As a last resort you could try a factory reset, but you will lose everything on the HD.
(The following is taken from satellites.co.uk)
- Holding Left and Right navigation buttons, switch power on and hold for about 30 seconds until Green play light comes on
- The 'play-light' will then rotate clockwise while the resetting procedure takes place. This may take up to 3 minutes. (this timing could vary if there are no programmes stored in the hard drive)
- It then reverts to 'Standby'
- Leave the machine for 2 minutes while hard drive reformats.