Meghann,
It's really tough to say what would be causing your problem, especially since you're moving around a lot and aren't always in a controlled environment. Perhaps someone else on these forums has had the same problems and can answer you.
What I would say is -- and this is no knock on Norton -- is that you can find a number of excellent free programs that, when combined, can cover most security holes. If you want to try a free antiviral product to see how it compares, I'd highly reccomend
AVG Free. Although the free version doesn't send you updates as fast as the for-pay version, I used it for a couple of years and never got a virus.
Just my two cents.