The only way that will "always work," so far as I've been able to find, is changing the user-agent string in the registry. That will, of course, affect every web browser instance running on that machine.
You might also try a Google search on DISPID_AMBIENT_USERAGENT
. From this Microsoft page:
MSHTML will also ask for a new user
agent via DISPID_AMBIENT_USERAGENT
when navigating to clicked hyperlinks.
This ambient property can be
overridden, but it is not used when
programmatically calling the Navigate
method; it will also not cause the
userAgent property of the DOM's
navigator object or clientInformation
behavior to be altered - this property
will always reflect Internet
Explorer's own UserAgent string.
I'm not familiar with the MSHTML component, so I'm not certain that's helpful.
I hope that at least gives you a place to start. :-)