  Using new HST/FOS, NTT/EMMI, and Keck/HIRES spectra of the
gravitationally-lensed double QSO HE 1104-1805 AB (z=2.31, \theta=3.2), and
assuming UV photoionization by a metagalactic radiation field, we derive
physical conditions (ionization levels, metal abundances and cloud sizes along
the lines of sight) in five CIV+MgII absorption systems clustered around z=1.66
along the two lines of sight. Three of these systems are associated with a
damped Ly-alpha (DLA) system with logN(HI)=20.85, which is observed in A. The
other two systems are associated with a Lyman-limit system with logN(HI)=17.57
(B). The CIV and MgII line profiles in A resemble those in the B spectra, and
span \Delta v=360 km/s. Assuming that the relative metal abundances in these
absorption systems are the same as observed in the DLA system, we find that the
observed N(CIV)/N(MgII) ratios imply ionization parameters of log\Gamma=-2.95
to -2.35. Consequently, these clouds should be small (0.5-1.6 kpc with a
hydrogen density n_H<=0.01 cm-3) and relatively highly ionized. The absorption
systems to B are found to have a metallicity 0.63 times lower than the
metallicity of the gas giving rise to the DLA system, Z(DLA)= 0.1*Z(solar). We
detect OVI at z=1.66253 in both QSO spectra, but no associated NV. Our model
calculations lead us to conclude that the CIV clouds should be surrounded by
large (>= 100 kpc) highly ionized low-density clouds (n_H\sim e-4 cm-3), in
which OVI, but only weak CIV absorption occurs. In this state, log\Gamma>=-1.2
reproduces the observed ratio of N(OVI)/N(NV)>60. These results are discussed
in view of the disk/halo and hierarchical structure formation models.
