I have been using Symantec Norton Anti Virus for years (Paid for!) but updating from 2005 to 2006 produced serious problems with my XP Home System that still haven't been sorted out. Windows Support failed with their first suggestion of a solution, and they now seem to be thinking a lot before trying again!
I can't access Windows Update or System Restore and User Accounts in Control Panel is just a blank window.
Norton refunded the cost of the update, but weren't interested in my problems.
I've switched to AVG Free with no problems with downloads so far.
ERB