Hi Hojo,
With service pack 3 applied to Project, do you see the option on the
Security tab to allow loading of legacy file formats? Can you please
confirm through Help About that you have SP3 for Project applied. Not
just SP3 for Office? In control panel with "Show updates" enabled in
Add/Remove Programs, I show two separate listings for SP3 one for
office "Office 2003 Service Pack 3 MainSP3" installed in March and
another listing under Project Office Professional titled "Project 2003
Service Pack 3 (SP3)

rojectSP3 also installed in March. I also show
the security update (kb948962) and security update for office 2003
(KB921598).
Again, with both the security update referenced in KB 948962 and
service pack 3 applied to MS Project, I have no difficulties saving as
HTML as long as the option, Allow loading of legacy file formats is
enabled. My version of Project (from Help > About):
Microsoft Office Project Professional 2003 (11.3.2007.1529.15) SP3
I am unfamiliar with the "Unblock word" issue you note so I can't say
whether this will fix Project or not. I doubt it.
I'm sorry, I don't know what else to suggest. Perhaps someone else
can weigh in with some ideas.
Julie