Hmmm, can you check the wood from underneath the step? While there is evidence of water spots, if the floors' not soft, I wouldn't worry. I'd put some penetrating epoxy on it and call it good.
On edit - I just read the other post you made with pics.... hmm.. are you sure it's not soft? I'd get a icepick or sharp screwdriver and really drive it in under the wall areas... if it is at all soft, then go ahead with the replacement (yes, you'll have to pull the inside skin around the area...).
If not, then, kill the area with bleach or wood treatment, then epoxy from there. I'd look beyond the windows for leaks though.. My guess is that it's leaking from above and traveling down the shell to the floors...