I purchased my home a couple of years ago and this year I am going to replace the deck and build a fence.  The deck was in terrible condition so I completely took it down.  To my surprise I found 2 below grade windows under my deck that had been dry walled over on the inside.  In fact the previous owner even left the glass panes in the windows.  I want to fix this permanently and fill it in so it won't ever be a problem again.  What is the  best way to do this?