Note that on some Macs, once you install a higher version of OS X the system prohibits you from re-installing an older OS version. I ran into this with my son's 2008 (Unibody) MacBook. He upgraded to Mavericks, then we upgraded his RAM using faster than advised RAM. It booted and seemed to run fine and fast, but alas, he began to have app and system crashes. Before determining that the RAM was the cause we assumed it was an OS corruption and he wanted to go back to Snow Leopard. We wiped the hard drive (after backing up to an external drive) to do a clean re-install and it seemed to go well up to 75% complete. Then it stopped and reported "additional resources not available" or something like that. Took it to the Apple Store Genius Bar and they said first, it's the RAM. Second, you cannot go back to SL after having had Mavericks installed. Seemed crazy to me, but several geniuses agreed that this is the case with many modern-era Macs.