I would try a clean install. Un-install, re-boot, shut down all non essential programs (especially anti-virus), re-install the game, start the game and see if you get an error, re-boot, shut down all non essential programs, install the
patch, re-boot, shut down all non essential programs, start the game and create a new character. This procedure is a bit over-cautious, but the best way to avoid install problems.
If that doesn't work;
Troubleshooting and Manual Install Procedure.
