1. Go find your Spore’s saved games. It should be located in your applications directory.

   eg. C:\Documents and Settings\<<USERNAME>>\Application Data\SPORE\Games

2. There should be a folder called Game0 (or similarly otherwise) as well as a folder Games0.old (again similarly otherwise).   Copy the folder Game0 to a spare location, as backup in case it doesn’t work.
3. Open the .old folder of the relating saved game you’ve just backed up.
4. Copy all the contents of this folder into the normal folder (eg. Games0) and overwrite all the files in there.
5. Load up and Spore, and check out when you saved your game previously.

Of course this may not work for everyone. Also it’s somewhat dependent on your saving habits.

Best of luck. Or not.

UPDATE: If you can’t find your “Application Data” folder. Go up to your menubar and go Tools -> Folder Options. From there go to the View tab.  Go down and look for the Hidden files and folders option. And select “Show hidden files and folders” from the radio buttons. That should then make the hidden “Application Data” folder visible.