With the most recent release of the VS 2008 SDK, HelpStudio Lite is no longer included in the SDK download. But it is still the tool we encourage you to use for help documentation authoring and you can easily download it directly from Innovasys.
I was doing a search on the download link just to see how easy it is to find HelpStudio Lite. It turned out to be harder than I thought (I used both Google and Live search). To make things easier, here is a direct link to it:
Hope that helps.