You should put blueskin on the framing before installing the window frame

Pull it all. Replace it with a vinyl window. If need be, flash it with aluminum and caulk all of the seams

Figure out why they are rotting and correct that issue first. Usual culprits are either lack of proper drainage or mis-aimed irrigation.

That outer area is entirely cosmetic so it's up to you whether you replace it or just rip it out and deal with what's left. It's the casement part around the actual frame of your window that's critical for sealing. The key to replacing those casements will be matching the dimensions of your window, because it's easy to shim a bigger opening but you can't squish the window into a smaller one. The windows are probably screwed to the wooden casements and sealed with clear or white silicone.

The most durable solution to replace what you have would be composite trim. It usually comes in white by default.

Pull out all the damaged wood, but more than likely that's going to extend into the frame around the window itself, at which point the easiest thing is going to be replacement windows

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Fixed this exact problem last year on my house. Replaced the rotten wood frame around my PCV windows and installed weeping tile and a sump pit outside.

Replace and Blueskin framing, get a PVC window installed... the pit is likely far too shallow and seems small for a window of this size, at least where I am from, good opportunity to add or replace weeping tile.

Outside foundation likely needs waterproof on a house that shows this kind of damage, not that they're necessarily related, but one is usually indicative of another in my experience...

Eavestrough issues could also be causing water buildup this close to the home, clogged downpipes or eaves. If your roof is old or you have trees in the area there is a good chance this needs attention also...

Couple of people have already said this but use PVC to rebuild the casing after you remove all of the rotted wood. Part of the problem there is it's too close to the ground. Another problem is concrete wicks moisture. Wood should never touch concrete. PVC with a polyurethane caulk will solve all of these problems

You can also build up one by six PVC. Stack it to make 2x6 stock. You can glue it together with plumbers glue or polyurethane adhesive.

Remove the whole thing and mortar in a glass-block window.

It’s wood below grade, it was destined to rot.

If you want to buy time treat with boat quality epoxy from West marine. It will have to be really really dry for it to work well. I have some garage windows with similar problems and they look great. The surface isn't newish looking but that fine for what I want. Alternative is replace everything with new high performance windows, there is a pretty good tax credit right now for insulation and energy saving upgrades.

Wood rots over time anyways, in most locations, even when drainage isn't an issue. So, there might hot actually be anything wrong. Could just be a moist are that has a lot of humidity. It looks like a block foundation, so the house likely has 30+ years, and that is likely an original window.

Vinyl is the way to go. If you're handy enough to be swapping out the casing, you're handy enough to put in a pre-built windowframe, and I think last time I bought one it was only in the $100 range, which is probably only slightly more than you'd pay for new casing lumber, caulk, and paint, and it'll last much longer.

Most of it looks okay. I'd pull the sill and replace with PVC.

Doesn't that remove an escape path in case of a fire?

A window this size isn’t an egress window to begin with.

Thanks for the reply. Just wondering.