More virus's on pil computer

Symantec updated and 3 more infections were found on the pc. I think these are remnants of the spyware loaded as the associated regkeys that these viruses would have installed were not found. Still its amazing how infected they were! Details are trojan.killav and
trojan.byteverify and
trojan.ecure.c. What really annoys me is the fact that these virus's are meant to have been detected since July 7th this year, september last year and 1999 for one of them, so how on earth did they get onto the computer in the first place? This is not the first time I've had this problem with Symantec and its a rather worrying trend how virus's are getting past its defences and yet they are either detected as old viruses or generic viruses that are again old and supposedly detectable.