Major Problems with Renault Megane 53 plate ?

Hi,
On tuesday night whilst out we got stranded as our car would not start - the whole electrics failed, AA came out after 3 hours jump started our car and said we need a new battery. we replaced the battery on wednesday night - all seemed fine. Yesterday, when we put driver side window down, we couldn't bring this back up! we've tried everything then ended up putting a plastic bag over it and car left on drive last night.
This morning when we tred to start car up we got the same problems as tuesday night car won't start again. we've spend £50 on new battery already! what can we do? Please help.