I suspect I'm out of luck on this one, but here goes...
Say I have a CHM help file that has http:// hyperlinks embedded in the help pages. When the user clicks on a hyperlink of the style:
<a href="http://www.example.com" target="_blank">click here!</a>
a browser window is opened and the target web page is displayed. If a browser is already open a new tab is created and the target displayed in that. If the user clicks on another link (or the same link) another browser window/tab opens, and so on.
Is there any way I can force all clicks of the links to go to the same tab/browser window?