Thanks for the great ideas fellas. I am sure with a combination of them I will get the screws out.
I tried an impact screwdriver and all it did was trash the slot, even with a tight fitting bit.
I just don't get why they are so stuck in there. The window frames are newer, and the ends of the screws aren't peened over and look like they should come out, but they are really jammed in there, all of them.
Now that I have source of replacement screws [thanks dabtl!] I am going to have at them. They are an unusual screw, with a wide and shallow head. Finding a replacement would have been very difficult without some help.