1. Check your hosts file (%windir%\system32\drivers\etc\hosts)
Clean out everything in there beyond this:
Spoiler
# Copyright © 1993-2006 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
::1 localhost
2. in the command prompt, type "ipconfig /flushdns" (without quotations)
3. Start your browser in safe mode (Firefox: there is a safe mode option in the start menu entry for it, IE there is a no add-ons option in Start->accessories->System tools)