First execute the downloaded DWFWriter3.5Setup.exe file but dont continue with the installation. The EXE fill will unpack into your user profile folder (c:\Document And Settings\<USERNAME>\Local Settings\Temp). There you can get the MSI installation file.
Once you extracted the DWFWriter3.5Setup.msi from the downloaded file just use the command line switch:
msiexec.exe /i DWFWriter3.5Setup.msi /qb!-
This will install it without user interaction - unattended.