virus definitions update

I just found out that my virus definitions has not been updated in six months eventhough they are set up to automatically update.  Everytime I manually tell it to update virus definition it starts downloading for  while then it stops and says an error has accord during update but when i click the one click support button It tells me that Norton is working properly.  However is still refuses to update definitions.  Could someone help me.  I have norton antivirus software 2010 my computer os is windows XP and i use broadband internet.  If anyone can help me I would really appreciate the help.