There is a product at sears you can use. Its a liquid with tiny tiny pieces of friction material in it.... You drip some of the product into the stripped screw and its supposed to give you 400% more gripping power on rounded off or stripped screw heads.

You could try to use a standard style (not metric) allen key somtimes the small increase in size is just enough to bust the fastener free.

or......... Just leave them in and put the others back!!! LOL!