Recentlly every time that laptop turns on and kde loads, nautilus will opened automatically.
I installed kde on ubuntu 11.10.
As default the KDE is trying to restore the previous session. KDE System Settings > Startup and Shutdown > Session management.
Try with the "Start with an empty session".
In other words, even though you close Nautilus before ending your KDE session. KDE is currently incapable of keeping track of this correctly and as such KDE will try to restore your Nautilus session even though it was closed before you ended your previous KDE session. This is a bug that will most likely not get fixed because of its low priority. So at present this is the only way to stop KDE from launching Nautilus at login.
On KDE System Settings > Startup and Shutdown > Session management, write "nautilus" on the field "Applications to be excluded from sessions:". It is better than selecting "Start with an empty session" because you don't lose the functionality of having you previous session restored.