Yes it is possible to change the battery on some models. If you're using a touch, however (which is what I'm assuming), then correct me if I'm wrong (someone), I don't think you can change that battery. But you definitely can on some models (I know I did to my 30GB Video).
You cannot upgrade an 8GB to 16GB, the drive space is soldered to the board.
I don't know of any significant viruses, but since it is a version of OSX, I'm sure it's possible (though improbable).