After rebuilding the carb on my '04, I decided to get a new battery for the bike too and kill two birds with one stone. I couldn't get the bike to start and ended up taking the battery back and trading it out, and that battery wouldn't work, so I went back. The guy at the store said those two batteries were from a "bad batch" and that this 3rd one would work for sure. I was able to get the bike to run for about 4 days and even rode around a little yesterday, and then today the battery died. Any ideas on what else it could be? The only other thing I've done to the bike since then is take out the left turn signal (it was cracked from falling over in a storm when I had it parked outside my boyfriend's apartment).

When I took out the turn signal all I did was disconnect where it originally connected and left it like that. Could that cause a parasitic drain?

To confirm it is the battery causing the no start problem, try jump starting it from a car or jump starter.

If you have a volt meter, a fully charged battery should read 12.6V with ignition off and not drop below 10v when the starter is cranking.

Starting with a fully charged battery and the negative terminal disconnected, note the voltage. Connect the negative terminal. Voltage should not drop at all.