2000 Mitsubishi Montero Alternator light stays on

While on my way to work, the +/- charge light lit up and stayed on. The next morning, I was still able to drive to work but when I was on my way home, the Airbag light came on and my radio turned off and the electric windows would hardly work. I was able to get home before dark and didnt have to use my headlights. I turned off the engine and when I tried to restart the engine the battery was completely dead. I dont think the battery is defective for I replaced it last spring. Do you think that the alternator is the culprit. Please advise