Dead battery

After being off-road for the past 3 months following back surgery, I went to start my VTS today and found the battery totally flat. No console lights, no alarm nothing. I've a new fully charged battery arriving soon but was wanting to know if anyone has had any experience of replacing a totally dead battery? I'm especially concerned about things like the immobiliser and air bag sensors being affected.
Any advice greatly appreciated.

Ta

if theres not even a glimmer of life left put jumper wires to spent battery terminals and to one with some life in it,then you can remove and refit with no problems,fingers crossed !!

Battery was fitted without any jumper leads - we figured if it was dead anyway then there wasn't much else to go wrong.
5 mins later all systems checked out, no warning lights on the console, radio worked fine and engine rumbled to life.
Thanks for the advice though.