Can't Delete Media, Plugins, or Themes

For some reason I can’t delete anything from my media library. I can delete the image from my post, but I can’t delete it from the gallery. When I try I get a page not found thing on my site.

The same thing happens when I try to delete a plugin, if I press the delete button, it doesn’t work, just a page not found. In order to delete the plugins I have to check them, then hit the delete from the drop down menu.

Also I can’t delete themes, it’s weird (same page not found). None of this stops me from using my site, it’s just really annoying. I’m new to all this and I installed wordpress with fantastico. Maybe that’s the problem.

Any help would be appreciated.