Diary Defender is a very popular and rock solid diary application, which has been around for almost 5 years now. It allows you to keep your diary securely on your computer, using password protection with DES encryption. Several people can keep their diary on the same computer, and Diary Defender has a nice login window.
You can store pictures with your entries, which gives your diary entries a more personal look and feel. Combine this with the ability to use the fonts you please, you can even change font, font color, bold, italic and underline wherever you please in your entry. While you are writing there are spell-checking available (if you have MS Word installed), and it has built in support for the WordWeb thesaurus/dictionary, which is also free software.
When you've written your entries, you might want to print them. This is very possible, it even allows you to print several entries at once, with automatic chronological order. Or perhaps you'd like to export the entries so that you can use them on your website? Diary Defender fixes that easily for you, it has built in support to export both to HTML and RTF.
It has lots of other features as well, but I am guessing you are very eager to try it out, so I won't hassle you anymore.