Infected even though Norton was up todate

Hi there,

 

I was infected with Backdoor.Tidserv!inf and Backdoor.Tidserv!gen5 over the last week while on vacation and using a non secure wireless netwoork for browzing and email (was only choice).  My Norton was fully up todate as far as I know (it looks for updates automatically).  My system had done a full scan several times over the last week with no problems reported.  It was not till I did a safe mode scan, did Norton pick up the virus.

 

As far as I can tell, this is not a new virus and the definition should have been included in updates I was getting - why did Norton allow my PC to become infected?  What can I do to stop similar occurances in the future.