How to convert a PowerPoint ppt file into HTML files?

You want to use pptHTML:

http://www.ma.utexas.edu/restricted-resources/utma-doc/xlHtml/pptHtml.txt

For debian based distros: http://packages.debian.org/unstable/utils/ppthtml

The C Source for the xlhtml package: http://prdownloads.sf.net/chicago/xlhtml-0.4.9.3.tgz

ppthtml is an executable installed through the same package.