[eluser]Dan Bowling[/eluser]
CodeIgniter doesn't really require anything all that special, and it runs great on IIS 6.
I'd just look up any normal PHP/IIS install guide (or run Microsoft's Web Platform Installer) and you should be good to go.
The only real trick is that you'll want to change your config.php file to use "index.php?" rather than "index.php" if you aren't doing any rewrites.