The reason earth spins is rather simple. It is because, it is falling around the Sun. As the Earth hurtles round the Sun, the Sun's gravity keeps it spinning, just as the Earth's gravity keeps a ball rolling downhill.
Ehm, no. It spins due to rotational inertia - and the tides are slowing it down! Yes, the days are getting longer (most of that time seems to be added around 16.30, don't you think?). As a matter of fact, I seem to recall that when human beings are completely isolated from outside time references, we fall into 23 hour sleep patterns so we're not completely keeping up with the world biologically.
Cheers,
/ft
(A longer explanation)
(Edited to add link)
[ 16 August 2001: Message edited by: ft ]