We used scanning tunneling microscopy (STM) to study the Cu-induced restructuring of Si(100)-(2 x 1) surface at the atomic scale. Copper was deposited onto silicon substrates kept at room temperature (RT) or 500 degrees C. Submonolayer amounts of Cu were found to be sufficient to induce a visible restructuring of the Si(100)-(2 x 1) surface manifested by the disappearance of the vacancy line defects (VLDs), as well as dimer rows disordering (for the RT deposition) or the formation of a c(4 x 4) surface reconstruction (for the 500 degrees C deposition) not reported for the Cu/Si(100) system so far. Higher amounts of Cu deposited onto the RT-disordered surface at 500 degrees C led to the formation of 3D CuSi crystallites, accompanied by a significant surface roughening. In contrast, deposition of large amounts of Cu onto a pristine Si(100) at 500 degrees C resulted in the formation of a near-(1 x 1) local surface atoms ordering.