I have had this problem before on my desktop with Windows XP, but running Live Update several times has eventually solved the problem. This time it isn't happening. I have run every scan I can find, even leaving it on all night to do a full computer scan, but still Protection Update not completing. I went into Security History and the last completion was on 20th Jan., but I also noted that it says Intruder Prevention is enabled and an unauthorised access had been blocked.
Any suggestions on how I can get Protection Update to complete?