how do i revert from yosemite to snow leopard on a macbook pro early 2008?

Thank you, I think everything is ok there. I can't believe a disc that has been in a case all this time is ruined, cleaned it anyway. Crazy idea, does yosemite disallow older discs to run? I remember having to make a startup disc out of a memory stick awhile back in order to erase and install. May have been for lion.

Yes other discs load so far the only discs that don't are the snow leopard install disc and the original OS X install disc do not boot.

You know what, I read those articles and now I'm wondering if I need a battery. I purchased an aftermarket that had good reviews awhile back but it bulged and was unsafe in no time. Since then I have been without and running with just the cord. I know I can't reset the SMC per the instructions without one. But does a lack of a battery cause such problems? Have tried other discs some work some don't. Burned dvd's so far do not (nothing pirated) but an old office 2004 disc launch as well as the dvd from the truck manual. Thank you again everybody or is it Y'all.

anymore ideas?