Do my PVC windows need re-sealing?

If the windows and doors close and the draft is not coming from the closed part then it must be coming from around the frames. Most windows and doors have a strip of plastic to cover where the frame meets the walls, piggle this off and see what it is like. If there are gaps then this all needs resealing. The best way to do this is either with expanding foam if the gap is more than 5mm or with a mastic and caulking gun if the gap is less. The expanding foam is fantastically messy so if you go down this route I strongly advise that you put a dust sheet under where you are working and masking tape around the frames. If any drops, leave it to set!!! Then cut it off with a sharp knife.