2002 Citroen C5 Engine wont start

My Citroen C5's battery went dead after not being run for 2 weeks and the freezing weather. After it was fully charged it did start. However when approaching a junction and slowing when it selected 1st gear and I attempted to pull away there was very little power and a plume of white smoke from the exhaust. Then power was restored and and it ran ok. When I got home it did the same thing was I went to pull on the drive. Since then it hasn't started at all. I fitted a new a battery and the engine very slowly tries to turn. When there was a smell of burning I stopped(10-15secs) as I didnt want to damaging anything else. Any advice would be very much appreciated.