The permissions for the kext are not set correctly. Running permissions repair won't work in this case as the faulty kext just gets ignored.
Open a Terminal window and try the following, assuming your kext is in /System/Library/Extensions as per the error message
sudo chmod -R 755 /System/Library/Extensions/AppleFan.kext
sudo chown -R root:wheel /System/Library/Extensions/AppleFan.kext
As far as I can remember, Leopard should rebuild the kext caches on reboot, otherwise you may need to remove these and then reboot again to force a rebuild.