BMW: Alternator or Battery?, free beats, 2000 bmw 323i

Question
I just got a 2000 BMW 323i and I'm having a problem with what I hope is just the battery.  The car will run fine for about a week and I'll hop in and it wont start.  The power does come on, stereo, lights the whole thing, but it will not turn over with out a jump.  Then it will go just fine for about another week.  Any idea what the proble could be?

Answer
Ryan,
 Based on the information you gave me, this could be either one.  If the battery is going bad, it wont hold a charge for very long, and if the alternator is going bad it wont charge the battery enough for for the ammount you are taking out of it (eventually running the battery down).

 That being said, obviously both items ARE working to some extent, but one of them (or both possibly) is going bad.

 You can run by the local autoparts store (namely Autozone) and they will test your Alternator for free.  Beats going to the dealership and paying their prices.

 Good luck,
 Josh