OS X :: My System Preferences Keep Resetting?

Every time I restart my iMac, my desktop wallpaper reverts back to the default and my mouse configuration is reset so the buttons no longer do what I demand of them (Button4: Expose, I say!)

It seems to be just those two issues but it happens pretty much every time the system shuts down (although not every single time). Energy saving is fine. My dock is fine. Nothing else seems to be touched.

I've reset the PRAM. I've deleted the system preferences file from the library (com.apple.systempreferences...). I've repaired permissions.

I still have this problem.

Any ideas or solutions, aside from reinstalling OS X or not restarting my iMac ever?

I should mention that I'm running 10.5.5 on an Aluminum 2.0 Core 2 Duo Intel iMac. My system is all-natural. No fancy hacks or system modification programs installed here.