Alternator problem????

its below freezing here in ny. My car would not start up, this happend about two weeks ago. for a breif second, two three lights would come on as i turned the key, but then it would die. The car did not even begin the ignition process.
so i took a guess that the battery wasn't working b/c its so old. I buy a new diehard batt. and install it. Start the car, car is fine
5 hours later, i get in, turn the key.....nothing. I got the lights to come on for a second, and then dead silence. same as before.

I'm guessing it may be the alternator, b/c a few mths before i noticed that the interior lights dimmed badly at low revs. Somethin the car did not do several months ago. I dunno, but if anybody has any idea of wht is wrong, help me out...

btw, i can't drive the car but im drivin the family minivan (oddessey, keepin it honda hehe:D ) but its not the same as drivin mah 3g, if u kno wht i mean.