I'd say you exceeded the serviceable life of those windows. Curious, sure appears to be significant corrosion, are you in an extremely salty (air) environment or subject to sea spray? It may simply be time to replace with retrofit windows. Pretty sure you can find some good YOUTUBE vids on the process, if so inclined. If your siding is stucco, the process is a bit trickier ... less forgiving, more easily damaged. There are basically two methods ... hack-out (skill saw) or frame collapsing tool (special).