Here is the code you can use to display HTML help. It uses the default browser in Windows but assumes netscape in UNIX and
requires it to be in the user`s path. 

 if { $tcl_platform(platform) == "windows" } {

      if {[file exists $env(COMSPEC)]} {
          exec $env(COMSPEC) /c start $EnvDir/help/envhelp.htm &
      }
} else {
      exec netscape $EnvDir/help/envhelp.htm &
}


-Greg Orchard 

See also http://mini.net/cgi-bin/wikit/InvokingBrowsers for more on invoking Web browsers from Tcl/Tk.