Try instead using

DTE.Properties(category, page)

which returns a collection of Property objects where you can change the
value for a given property name.

You have to guess the names of category and page, which in you case is quite
difficult but here you have some hints:

- The values for Text Editor / HTML/XML / General are stored in registry
key:

HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\7.1\Text Editor\HTML/XML

And you can retrieve them with

       Dim colProperties as EnvDTE.Properties

       colProperties = DTE.Properties("TextEditor", "HTML/XML")

- But your value is are stored in registry key:

HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\7.1\Text Editor\HTML
Editor

but I have been unable for 10 minutes to get the correct category and page.
I suppose that you can always change the registry directly...

- Here you have the docs about category and page names:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vsintro7/html/v
xgrfdtepropertiescollections.asp

But yours is not in those lists...