Grrrrr (a.k.a. why?)

Why does it seem everywhere I go, car trouble is soon to follow. Just yesterday I spent $210 getting the windows tinted in my Sentra, and today I pick up a custom made dash cover to the tune of $30. Last night leaving from the movies I get in my car, put the key in, turn it, and I am greeted with the sound of absolutely nothing. Battery was completely dead. Didn't use the lights, so that is out of the question. Get a jump start, but the battery is so far gone that when I go to turn on the A/C, lights start cutting out. Since I just had the windows done I had to leave them up. This meant I had to drive most of the way home with no air. When I got home and locked it up I noticed the dome light was staying on. I checked the setting, it was in the middle position which means it should turn off when I arm the alarm. I tried again and it still stayed on. I flipped the switch to off, but it didn't make much use, the battery was still shot. As I type this I am trying to get some life back into it, if not then I need to get a new battery and find out what the problem is and see if it is under warranty.