Downgrading OS X version from Mavericks to last OS upgrade?
The last Mac OS which supported Power PC emulator (Rosetta) on x86 was Snow Leopard. You can back up your data using Time Machine, find distributive on your CD and install it.
UPDATE
As people said, you should copy your data directly without using Time Machine.
A simpler solution: you may be able to run Snow Leopard in a virtual machine. A quick Google shows that running Snow Leopard in Parallels under Mavericks should be possible. Here's the first link I found, for example.