I'm putting this here mostly because I ran into it yet again, and I wanted to remember for next time.
I installed IIS on a computer, then installed ASP .NET 2.0. I then installed my default website (in this case, it's a product called Brocade FLM). Lastly, I went into the website properties in IIS Manager and set ASP.NET to v 2.0.
When I went to http://localhost/FLM, I got a standard 403 (page cannot be found) error. When I went to http://[my computer IP]/FLM, I got the login prompt, but it redirected to a 403 page.
In IIS Manager, click on Web Service Extensions.
In the left pane, select ASP .NET 2.0 and click "Allow" to enable those extensions
I'm now able to load my website at http://localhost/FLM no problem.
Not rocket science, but I had forgotten and must have spent 10-15 minutes on this.