if i were you, i'd avoid Kazza like the plague, its fairly secure against hackers i think but it is riddeled with awful spyware. See if u can get hold of some of the better clones like Sharezza (sp?) of K lite K++ (again sp?) that hook up to the same network but dont have the same bagage as official Kazza.
I don't think they will leave u open to hacking, but just like the usual internet if you download suspect stuff then u are asking for trouble. That said, i think Norton should probably keep you protected.