it may be a virus that is disabling your Norton, try one of the free online scans,
Trendmicro link
PandaActivescan link
or it may be a corrupt windows file, to fix this place your XP (assuming you are using XP) CD in the cd-rom drive, go to start > run and type in sfc /scannow (note the space between c and the /)
This will replace any corrupted windows files.