He there,

i recently switched to kubuntu 10.10 + kde 4.6 ... and I have seen it more than once by now, that "kwin" wont start properly. So after login, any auto-opened application will not have the decorator bar; mouse, keyboard dont function properly.

When that happened with gnome; i would launch the fusion icon and reload compiz. I had hoped that KDE does better ... anyway: i tried launching kwin from a konsole once, didnt really help.

so, any idea what to do?

you can use fusion-icon to reload kwin

since this happens both in Gnome/Compiz and KDE/Kwin I'd look at your graphics card and drivers as the potential cause - what are they?.

This sounds like Compiz gets also started in KDE. If KWin gets started and does not work, there would be at least a crash dialog. Furthermore you would not be able to do anything - without window manager nothing is working.

KWin has a rather good crash handling. It gets automatically restarted and after four crashers in a row it offers to launch a different window manager. From your description it really sounds like it's not KWin