Several ways of getting stuck screws out. That is a torx and sometimes people might have used a hex bit to tighten it some and stripped it.
If you can get to it with a very small, and very sharp center punch, you might be able to tap it in the counterclockwise direction with a small hammer and get it started loosening up.
Another way is using a Dremel rotary tool with the smallest cut off wheels they make to slot the screw and then use a common screwdriver to back it out. If you try that, be sure to wear a face shield or at least goggles, because those small cutoff wheels are bad about exploding and sending pieces flying everywhere. Don't try running the Dremal full speed. Cut the RPMs down with the adjustment wheel and it will help keep the wheel from jumping and grabbing. Just set the RPMs where it barely throws sparks and slowly, very slowly, ease it into the metal.
After you tried all that, the other possibility is to drill the head off it. Exact center, with a bit just about 3/4 the diameter of the full screw head. Center punch exact middle of the screw head first. Drill in just a short ways, and the head will pop off. Then after you get the windshield out of the way, maybe you can get hold of what is left of the screw shaft with some vise grip pliers or small channel locks.
If the screw is galled on the threads. you gonna have to tap the thread with a good set of taps the proper size, and you may even have to drill the galled threads out to the next size up and put a larger screw in to put it back on. Whatever you do, use a blue Locktite on it when you replace it. The Locktite not only helps keep it from backing off, it also helps keep the threads from welding to each other from over torquing. You can take one of the other screws with you to a real hardware store, as opposed to a huge home improvement store, and they will be able to find the right taps.
There is usually no easy way to deal with cross threaded screws or bolts. Whatever you do to get it out may mess up the threads, but the threads might already be messed up and that is why it won't turn.
