After some googling, it looks like this may be a WEBrick specific error with Windows and IE (you didn't list the browser(s) you tried, but most results also list IE as a culprit). I would try:
- Hitting
http://localhost:4000
from Firefox/Chrome and see if the error persists - Changing your default web server from WEBrick to something else (I'd recommend the Ruby Thin Server). An easy walkthrough specifically for Octopress and the Thin server can be found at this blog post.