Apostrophes and other special characters showing strange code after upgrade

Hi, i’m experiencing an error with special characters and apostrophes, etc…after an upgrade from 2.3.3 to 2.5.1. Based on info from other threads i found, i’ve tried both commenting out and deleting these 2 lines, which were not in my old 2.3.3 wp-config but are in the current 2.5.1 one:

define('DB_CHARSET', 'utf8'′);
define('DB_COLLATE', '');

But when i take them out i get the following error:

“Warning: Invalid argument supplied for foreach() in /home/content/mywebsite/html/wp-includes/classes.php on line 92”

And all my pages and posts show page not found errors.
I’ve also jtried, without success, changing it to:

define('DB_CHARSET', 'UTF-8'′);
and
define('DB_CHARSET', ''′);

I’ve also tried the following plugin, but it made no impact at all:
UTF-8 Database Converter

My old 2.3.3 i’m pretty sure was set to UTF-8 in the admin, as is the current version. The header file in my theme has always set the coding of the page to utf-8 as well, so i’m not sure why this is happening and am unsure where to procede from here.

The garbled code that is showing up looks something like this:
– Instead of network’s it shows network’s
– Instead of “Please Help Me” it shows “Please Help Me”

Any help would be very much appreciated!