While trying to test a web site on a local (development) machine, I ran into this error from ASP.NET:
unexpected error 0x8ffe2740 occurred
Simple dialog, "OK" button is the only option.
I found this blog post, where Lorin Thwaits suggests that some other program is already controlling port 80, and then gives an easy way to find the culprit and kill it.
As with most computer problems, this one was user error. I forgot that I had the JungleDisk monitor running (and didn't remember that it uses port 80 anyway!). With JD killed, IIS is fine, and serves up sites just fine.