As a professional who deals with this kind of thing on a daily basis, I can say your experience is very common. Get rid of Norton (Uninstall from Control Panel) and try AVG, Avira, NOD32 or the like. 
Why it works for a while then stops is anybody's guess. But this is the usual pattern. 
BTW, McAfee is nearly as bad.
                
 Lantash
                
 March 2007