How do I unscrew the 38-090?

Thanks for getting in touch with us. I've run across this issue every now and then. Our only guess is that the original makers of the window never intended this lock to be replaceable, which is frankly bad design. Your only option would be to maybe see if you can use a long neck #2 Phillips screw driver. If that doesn't work, you could try to notch out some of the vinyl, or drill a hole large enough to stick a screwdriver through. 

Sorry we can't offer an easier solution!