WikidPad is a “personal” wiki that runs in Windows. It is free and open source.

I have been using it for several months now and find it very easy to use. My only complaint would be that it is a little slow.

One of the best features (and one that I have not seen in similar applications) is a very customizable tree view of the pages in a separate left panel. I change the graphic and text colour to distinguish the different pages and it makes navigation a lot simpler than clicking through the links.