I agree with madhouse that using Powerpoint is a good place to start. If you have the latest version (that comes with office XP) you can use the Organisational chart to draw a hierarchy of web pages that look pretty good.
Also check out the flow chart features in words drawing menu. In the latest version you can draw lines between boxes that are actually joined to the boxes. If you link two boxes and move one of them, the line still links them.
Damn, I sound like I work for Microsoft...
As well as drawing a chart that shows the structure of the site, you should also draw simple layouts for the pages that show where logos, navigation, content etc are going to appear on the page. You don't have to actually show what the navigation etc will look like, but it is a start that your client can get a handle on, and can give feedback on (though make sure they know that this is just a guide to layout - people get very confused about such "blueprints"
An alternative to this is to start creating the web site now. Don't worry about design at this stage. Make the folders and pages that you know will appear on the site, and link them together as they will be linked. Then add descriptive text to each page saying what will be on it. You can then give your client access to this skeletal site, as well as simple architecture diagrams. You might find that a web site itself is the only documentation you need - and by creating such documentation you are also doing some of the work that needs doing later anyway.
Finally, O'Reilly do a book called "Information Architecture for the World Wide Web" which covers a lot of ground. I have the old edition, and it is very old, but has a lot of sensible advice and covers dealing with clients too. Haven't seen the 2nd Edition, but it has had good reviews.
Hope this helps... --
Dunx
