Why would Renault Megane '99 Petrol not start after battery changed?

I'll keep it short:
My Petrol Megane I Classic from '99 got frozen and battery died. I've change the battery and..
- the car would start for 1 second and then it stops
- if I try again imediatelly, is only trying but it doesn't really start at all.
- if I wait about 10 minutes or more, I get back to step 1, when the car starts for 1 second and then it stops.

The imobiliser behaviour is as follows:
- before starting is flashing once a second
- when I turn ignition is stays on for 1 sec and then starting to flash quicker then ones a second.

BTW, few hours later: now the engine wouldn't start at all, even if I wait 1h. Would only try, starts to "rotate" but won't actually start. (same as if I would have waited the 10 minutes in my previous post)

I suspect has something to do with "feeding" of the engine.

Any recommendations are welcome.
If you need more details, please ask.

Scorillo
February 2011