Well, see this forum post:

http://forum.mandriva.com/viewtopic.php?t=91540

KDE should really handle the situation where desktop effects are
enabled but the driver in use does not adequately support them better.
It shouldn't just completely fail to work. Surely it should be possible
for it to fallback smoothly - or at the very least give you some kind
of emergency bailout that says "desktop effects aren't working, do you
want to disable them?" - rather than just crapping out. Can we do
anything about this, KDE folks? Thanks.
--
adamw