Ya that's what I was talking about.
How big is this timber and how deep is the rot or how much is left.
Is there anything between timber and concrete, tar paper, roofing material, anything?
Condition of wood higher under the window?
Condition of the end of the floor joists, and are they sitting on concrete or are they only nailed in place?
The floor does not extend over this timber, the stud sits right on the timber, yes?
Poke around with a screwdriver, start with a really solid peice of old wood and you will be able to judge how soft this stuff is?