The machine that I was working on today had a few stripped out screws. Many ATVs and motorcycles have parts put together with small phillips head machine screws. The screws are typically steel and they are threaded into an aluminum piece. Steel and aluminum together leads to corrosion and stuck fasteners. If the fastener is a hex head bolt it is normally not a problem, but phillips head screws strip out very easily.
The first thing I do to remove one of these is tap it on the end with a hammer and punch. Sometimes the shock of tapping on it will break it loose. If the head strips out a little you can get another chance at it by peening the cross closed with a punch and hammer, just hit it a little harder than you did the first time. If you are careful the sides of the cross will bend over a little and make for a tighter fit on the screw driver. The screw driver may need to be tapped into place.
If the previous steps don't work (or someone else has already wrecked the screw like my project for today) you need to take more serious action. On most parts you can grind or drill the head off the screw and then use vise grips to remove the left over stud after the pieces are taken apart. I like to use my set of left hand drill bits for this job. Frequently when doing this the drill bit will catch on the screw and the torque of the drill will back it right out.