  We report the discovery of a Compton-thick AGN and of intense star-formation
activity in the nucleus and disk, respectively, of the nearly edge-on superwind
galaxy NGC 4666. Spatially unresolved emission is detected by BeppoSAX only at
energies <10 keV, whereas spatially resolved emission from the whole disk is
detected by XMM-Newton. A prominent (EW ~ 1-2 keV) emission line at ~6.4 keV is
detected by both instruments. From the XMM-Newton data alone the line is
spectrally localized at E ~ 6.42 +/- 0.03 keV, and seems to be spatially
concentrated in the nuclear region of NGC 4666. This, together with the
presence of a flat (Gamma ~ 1.3) continuum in the nuclear region, suggests the
existence of a strongly absorbed (i.e., Compton-thick) AGN, whose intrinsic
2-10 keV luminosity is estimated to be L_{2-10} > 2 x 10^{41} erg/s. At
energies <1 keV the integrated (BeppoSAX) spectrum is dominated by a ~0.25 keV
thermal gas component distributed throughout the disk (resolved by XMM-Newton).
At energies ~2-10 keV, the integrated spectrum is dominated by a steep (G > 2)
power-law (PL) component. The latter emission is likely due to unresolved
sources with luminosity L ~ 10^{38} - 10^{39} erg/s that are most likely
accreting binaries (with BH masses <8 M_sun). Such binaries, which are known to
dominate the X-ray point-source luminosity in nearby star-forming galaxies,
have Gamma ~ 2 PL spectra in the relevant energy range. A Gamma ~ 1.8 PL
contribution from Compton scattering of (the radio-emitting) relativistic
electrons by the ambient FIR photons may add a truly diffuse component to the
2-10 keV emission.
