One of the most frequent causes of failure for a Wordpress installation are the wrong permissions set on your document root directory. This is due to the fact that most hosting companies set such permissions in a way that simply blocks Wordpress (in some cases) or, more frequently, causes several side effects which are not immediately noticeable after the first installation. These side effects include broken permanent links, a plethora of HTTP 404 errors and, sometimes, broken RSS or Atom feeds. Fortunately, the solution is pretty simple.
You should set your permissions to 755 for your document root and all the included Wordpress directories. Here's how the 755 code looks in an FTP client:
If you've access to a web control panel (such as cPanel), you can set the proper permissions from there. Always remember to include also the Wordpress directories in a recursive manner.