Norton updated this morning and a huge blob of these medium-class warnings started appearing in history, but only when opening Internet Explorer. They say:
Unauthorized access blocked (set file attributes)
Actor: SVCHOST.EXE
Actor PID: 936
Unauthorized access blocked (set file attributes)
Actor: IEXPLORE.EXE
Actor PID: 4236
Someone else reported this same problem on the 360 section. It started at roughly the same time for him. Could an update have caused this? It was absolutely fine until sometime after midnight.