I hate to make a new thread for something that should be simple to find an answer for, but my google-fu is not strong enough today...

My 2001 Bora 4mo has been sitting on the street for a couple weeks with a flat battery. Lights got left on, and was too busy with life to be able to spend time charging it up. A mate came around with a starter last week, but it couldn't deliver enough juice to get the engine turning.
Haven't tried using jumper leads yet, as I don't own a pair.

I figured with the battery being dead for a couple weeks, there'd be enough sulfation to cause problems, so a new battery may be in order.
I've heard that disconnecting the battery will stop my (OEM, tape deck (woo!)) headunit from turning on, and I'll need a code to get it working again.

How do I get the code? Or is this headunit too old to need one?