The initial stages of chemical vapor deposition of ZrO2 from zirconium tetra-tert-butoxide (ZTB) on Si(100)-(2x1) have been studied by Scanning Tunneling Microscopy (STM) and synchrotron radiation excited Photoelectron Spectroscopy (PES). The STM images and core level (PES) spectra indicate that the predominant surface modifications induced by ZTB are due to silicon carbonization and formation of zirconium dioxide. The carbonization reaction leads to formation of subsurface carbon and two types of reconstructions are discussed: dimer vacancies and dimer vacancies in conjunction with a rotated surface Si-dimer. Indications for the formation of small amounts of zirconium silicide are also found. No evidence for silicon oxidation can be observed with PES, in contrast to the interface properties previously found after larger exposures to ZTB.