I actively use a few blogs I've set up, most on WP1.5x and one on 2.0. Recently though, regardless of which blog/admin I'm using, whenever I post a new entry or edit an existing one I get a bizarre Windows generated message asking me which program I'd like to use to open the file 'post.php' and treats it as if it's a file to download. When I check, the post has been added or edited suitably and everything else works fine, including of course every other php page I use in WordPress or otherwise. I'm guessing the page post.php should redirect me somewhere but isn't doing.
Any ideas what's broken?