My permalinks broke when I upgraded to WordPress version 2.6. This means that when you click on the title of a post, instead of seeing that particular post and its comments, you see a 'file not found' error message.
The fix took an hour to find, and a minute to execute. In the settings area, I needed to add a word of text to the category and tag bases. I added the words "category" and "tag", and the problem was immediately corrected.
Huge thanks to Otto42 on the WordPress Forums!
Your getting there slowly but surely Realtor,