Just spent several hours working on someone's Vista Home Premium x32 SP2 computer whose computer would not boot up properly in normal mode. There was hard drive activity after the POST screen and then the screen would go black and computer just sat there. Was able to boot into safe mode and ran MSCONFIG to use minimal bootup in MSCONFIG and boot into normal mode. However if MSCONFIG was set to normal mode the computer will not boot.
Uninstalled Norton Security via safe mode and was able to boot into normal mode. Downloaded the Norton Removal tool and rebooted. No problems. Windows is fully up to date and ran a competitor's online virus scanner and showed no infections. Reran the Norton Security installer from the download folder and ran liveupdate. Update installed fine and rebooted into normal mode without problems. Ran liveupdate again showed no updates available. Rebooted Vista and the computer refused to boot into normal mode.
These are exactly the same symptoms that the person experienced themselves!