  We present mid-infrared (MIR) observations of the Type II-plateau supernova
(SN) 2004et, obtained with the {\it Spitzer Space Telescope} between days 64
and 1406 past explosion. Late-time optical spectra are also presented. For the
period 300-795 days past explosion, we argue that the spectral energy
distribution of SN 2004et comprises (a) a hot component due to emission from
optically thick gas, as well as free-bound radiation, (b) a warm component due
to newly formed, radioactively heated dust in the ejecta, and (c) a cold
component due to an IR echo from the interstellar-medium dust of the host
galaxy, NGC 6946. There may also have been a small contribution to the IR SED
due to free-free emission from ionised gas in the ejecta. We reveal the
first-ever spectroscopic evidence for silicate dust formed in the ejecta of a
supernova. This is supported by our detection of a large, but progressively
declining, mass of SiO. However, we conclude that the mass of directly detected
ejecta dust grew to no more than a few times 10^(-4)Msun. We also provide
evidence that the ejecta dust formed in comoving clumps of fixed size. We argue
that, after about two years past explosion, the appearance of wide, box-shaped
optical line profiles was due to the impact of the ejecta on the progenitor
circumstellar medium and that the subsequent formation of a cool, dense shell
was responsible for a later rise in the MIR flux. This study demonstrates the
rich, multi-faceted ways in which a typical core-collapse supernova and its
progenitor can produce and/or interact with dust grains. The work presented
here adds to the growing number of studies which do not support the contention
that SNe are responsible for the large mass of observed dust in high-redshift
galaxies.
