Hello flynn10.
Sorry to hear of your problems.
Why? ... As everything is now erased I doubt that anyone can tell you. But it seems likely that something got past your defenses, which included Norton. This is unfortunate but not unknown and I believe it can happen with any protection software as the threats evolve. You have to stay up to date and stay alert and even then something can get through.
Had you come here before the reformat and reload there are some very knowledgeable people around who may have been able to help. Sadly that does not help now but it is a good reason to have a good backup which hopefully you have, from Norton 360, or elsewhere. However there may be problems in that backup so if you have one and are thinking of restoring to your recovered system you may like to post some details to see if others can offer advice on how to avoid a re-infection.
I hope that helps.
Better luck for the future!
[Edit - sorry for the double post with cgoldman.]
