From XDA-Developers (this seemed to work for a few people, including me):
DLL error solution:
* Use UK version of Kies - ends 11_3, running "As Administrator".
* If you use a password / pattern lock, temporarily disable it.
* Start process - as the phone restarts, you'll see Windows try to install a driver but not quite succeed before the DLL error occurs.
* DO NOT CLOSE KIES.
* Restart the phone.
* Wait until it's completely restarted, then reconnect it to the PC.
* Kies will attempt the firmware update again and this time it will work, with the driver already loaded into memory and installing quickly.