Until IE8 isn't possible to create extensions with only html and javascript.
To more information about all thing possible see this
http://msdn.microsoft.com/en-us/library/aa741312(VS.85).aspx on Extending Internet Explorer and Browser Extensions section.
But you will need to learn how to develop COM components with c/c++.. It's not so easy if compared to html and javascript.
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Murilo Maciel Curti (Shinji)
Software Engineer
Microsoft Brazil Windows International Team
http://blogs.msdn.com/winbr - Windows Brazil Team Blog
http://shinji360.spaces.live.com - Personal Blog