My problem began as a error similar to that described in thread #66321: each time I publish from Write Post, Firefox 1.5 tries to open post.php, but most times publishes the post. In IE 6, 'Page cannot be displayed' displays when I try to publish, but post is still published. Inconvenient, but I can live with it.
Now this happens when I try to log in, change themes, or try to change options, my browser tries to open the corresponding *.php. Now sometimes the post does not publish, the theme can't be changed, and I'm not able to log in.
I did deactivate all my plugins; this had no effect on the problem. (It's an intermittent problem, so its hard to be sure)
I have no idea where to start trouble-shooting this.
Advice would be greatly appreciated. I am new to WordPress, so please explain gently if you can.