>>1
I had the very same issue. I could not post on img.4chan.org at all. Not that I really cared since I don't frequent /b/ or /r9k/ often. I had to edit my hosts file because I added 4chan server IP and domain addresses since the site was experiencing downtime. I forgot I had left all that stuff in my hosts file hence the "400 Bad Request" error I kept receiving.
Just goto Start, run, and put in "%SystemRoot%\system32\drivers\etc\hosts" and see if there's anything that was added into your hosts file. You can open it in notepad.
It should look exactly like so when you open the hosts file (or at least if you open it on Windows XP, I'm not sure about Vista or 7):
# Copyright (c) 1993-1999 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
#
# For example:
#
# 102.54.94.97 rhino.acme.com # source server
# 38.25.63.10 x.acme.com # x client host
127.0.0.1 localhost