Specify an HTML file as the original and convert to a DOC format with the /C switch set to 101. On-screen confirmation with the /V switch:
ConvertPPT /S "C:\HTML\sample0003.htm" /T "C:\PPTOutput\s0003.doc /C101 /V
Note it is highly encouraged that you use the Verbose (/V) switch initially to see what the status of your conversion is.