Try some penetrating oil or WD40. Depending on how stripped the screws are you can:
1. Tap the screwdriver with a hammer to help get the bit to bite better. Use the correct size driver bit.
2. If that fails, slot the screw with a dremel or use an EZ out kit.
If it's really rusted and stuck, the screw will break very easily. Definitely use a rust cutter first. If it does break, you can probably grab the residual shaft with vice-grips and back it out.
PB Blaster is really good at freeing up stuck screws/bolts. Just drench your screw in it and leave it for 24 hours, hammer it a couple times to try to break the rust free. You can get it at any auto parts store.