All AT&T @Home users are now AT&T Broadband Internet users. They switched us over to essentially AT&T WorldNet over Cable. Excite@Home was just an ISP that got exclusive rights to AT&T's coax. (Like MSN or EarthLink over Verison DSL.) Some areas are still AT&T RoadRunner areas, and others...
If I remember correctly, when I bought my copy of OS X, there were three proofs of purchase inclosed in the box. I'm guessing you'll have to take one of those with you to the Apple Store, CompUSA, etc. But, YMMV.
I first used root as my first user when I installed OSX. Better off not doing it. I just reformatted, reinstalled and used some other name and all was okay. Root is already preconfigured by the BSD undercarriage and I think using it again just fuxors it all up. Why? You got me, just don't...