Until the next time it rots and needs replacing again. To fix it properly, you need the window out completely and a new wooden frame fitted to do it properly. I had the same problem here, but with all of the windows and it was cheaper just to replace the lot with new, rather than mess about and quicker.
if its possible can you post pics of the whole window from outside and inside and show whats below the opening in , is it a bathroom?
you could remove the pvc frame and clean it upeasy enough and then clean out all the surrounding sub-frame.
with pics we could suggest what to do next.Maybe a new sub-frame and cill?
You shouldn't have started another thread, it just confuses. Looking at the other picture it looks like an old timber sash window has been removed and replaced with pvc and the inner reveal filled with botchy timber. Just chop it all out and fix some new timber in and cover it all up with a PVC window board glued on with no-nails and seal it all with silicone.
This is a bit of a bodge, but a proper repair is invasive and time consuming - probably something to save for when the bathroom is next redone.