I think Hina means that getting rid of the wood frame the window is going to fit in is the best possible thing to do.
In my case, I had PVC box windows installed into existing wooden frames. BUT, I made up plastic (PVC) flashings that I caulked onto the bottoms of the PVC windows to act very much like a "roof" to keep the rain and snow melt water off the wood. Those PVC flashings stick out over the bottom wood frames a good 1/2 inch so it protects them from both Sun and rain. Also, the PVC flashings are about 1/2 inch above the wood frames so that if that wood ever does get wet, the flashings will allow that moisture to evaporate without allowing any rain or melt water to get that wood wet again.
I'm convinced that if you take reasonable measures to protect an existing wood frame from getting wet, then installing PVC box windows into existing wood frames is OK, but you have to make sure you protect the wood at the bottom of the frame from getting wet.