NIS slowing down Microsoft backup

I back up my lap top hard drive in full to a Toshiba 320GB external hard drive using Microsoft Back up. But recent back ups have been taking a very long time. 17 hours for 55GB !!! This problem seemed to to start around the time I installed Norton Internet Security 2012 & Performance pack. Is Norton slowing the data transfer down and if so how do I stop it happening. Any advice would be very welcome.