Cookie Won't Delete

I'm trying to setup a simple user authenication for my site. I'm using cookies to pass the credentials between pages. But the function header() is giving me problems logging in and logging out. As you can see in the code below, I try to redirect the user to a page after setting or deleting the cookies. The scripts redirect okay, but the user never logs in and never logs out correctly. The scripts works correctly (logging in & logging out) only if I comment out the header() function.

Here's my code for setting the cookie (login.php):