We have a wiki running very well under IIS.
The message "Invalid access to memory location." is not IIS related. It indicates that the mysql extension to PHP tried to access a segment of memory that is not valid to it. It may indicate a corrupted DLL file, mixed versions or other configuration errors.
I also suggest checking if the php.ini that is shown in phpinfo is the one you're editing. This is a common mistake.
Another thing: are you sure you need to load php_mysqli.dll? I'm not familiar with this extension, but I use only php_mysql.dll.