The command prompt option can be used to check that the drive Vista is installed on is actually visible. When the X:\Sources prompt appears, type c: and press Enter. If the drive is accessible, the C:> prompt will appear. If this is the case, type chkdsk / r and press Enter to verify the drive. Press N when asked to forcibly 'dismount' the drive, then Y to schedule the drive check the next time you restart Windows. Close the dialogue box and click Restart. Ignore the prompt to boot from CD and the Disk Checking tool will hopefully fix the problem.