How to remove the rusted Pivot Bar

I have repaired some of these window sashes in the past, but it was very difficult. I'll try to help as best as I can.

You say screw head is stripped. What I did was cut a slot in the head and use a slotted screw driver to remove it. Now, on the pivot part itself is where the tricky part comes in. You will need to use a heat gun and to loosen up the pivot bar. Heat it an tap the bar into the frame just a little to loosen it up, then use a pair of long-nose pliers to pull it from the frame. Please be very careful not to use too much heat! This is important. You could damage the glass or warp the vinyl frame. It might be better to remove the bottom rail from the sash entirely. To do this, you must remove the side rails first.