Perhaps try downloading and saving a copy of the installer to a different PC. (If you're on a 32-bit Windows system, try to download on a different 32-bit Windows system.) Copy the iTunesSetup.exe installer to an external hard-drive, or a large-enough thumb drive or burn it to a CD-R. Carry the installer back to your PC, copy the installer to your hard drive. (Don't run the install from the ExHD, thumb or CD-R.) Start the install by doubleclicking the iTunesSetup.exe file.