Flat battery

My car's battery is flat. Its a newish battery, ~ 3 months old, and whatever's been draining the battery has had 4 days to do it, as I haven't used the car since Sunday. There isn't even enough juice to illuminate the interia light. I've checked all the light switches, and they appear to be off, so I hope - touch wood - that I left the radio on. I really hope it was the radio. If its not, I still have the old battery, so at least I can easily check if its a fault with the battery, or if its something in the cars electrics. I really hope it was the radio.

Suffice to say, a flat battery means that I will not going out tonight.