CHtmlView Windows 7 issue

Hello,
I have a (maybe small) problem that is driving me mad.
I am using a CHtmlView derived class to display a html file that is stored locally on the PC. I'm using Navigate2() for that. On Windows XP machines everything works fine but Windows 7 always opens a new IExplorer window instead of displaying the file in my view. I tried to override OnNewWindow2() but the according event does not occur.
I use the following code in the OnInitialUpdate() function of the view:

CString fileName = theApp.m_sSettingsPath + CString("Certificate\\certificate.htm");
Navigate2(fileName);

I've also tried the "file://" syntax, but same behaviour.

Thank's for your help,
Ralf