Sometimes that happens when you are infected with malware and it does not wish for you to fix your computer. Here is what I would do first:
1) Check you HOSTS file (usually located at C:\Windows\System32\drivers\etc
Domains listed there may be blocked, you can delete everything and restore to a clean HOSTS file. Here is what mine looks like right now:
# Copyright (c) 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
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
# For example:
# 188.8.131.52 rhino.acme.com # source server
# 184.108.40.206 x.acme.com # x client host
2) Check for changes (usually by malware) to your DNS settings. Sometimes malware will do this and send you to malicious DNS servers. Ask your ISP for help with this as they will have to tell you what DNS servers you should be using.