Well, it's your choice if you want to keep Norton or not. As I said, Avira is for free, so you won't lose anything though - except if you choose for the Avira premium version, but then again, it's really cheap if you compare it with Norton.
For avira premium, if I'm not mistaken, if you select to purchase that version, you can select the currency (dollar) in your case.
But if I were you, I would just try the free version now and see if you like it.. then you can still decide afterwards if you want to purchase the premium version or not. I have the premium version installed on 2 computers and like it a lot
Spyware doctor - to be honest, if you didn't purchase it, I would uninstall it. It's not a bad scanner though, but not really one of my favorites. But then again, that's my personal opinion
Please read my Prevention page
with lots of info and tips how to prevent this in the future.
And if you want to improve speed/system performance after malware removal, take a look here
Extra note: Make sure your programs are up to date - because older versions may contain Security Leaks. To find out what programs need to be updated, please run the Secunia Software Inspector
Happy Surfing again!