Sometimes infections can make changes to a file called hosts. Windows uses this file for sending you to the site address that you enter in your browser or denying you the access to these websites. But infections can add security sites that offer removing instructions for certain infections,update servers so you can't reach them in other words you can't update your security solution anymore.
With Software
You can solve it by downloading this tool.
http://www.funkytoad.com/download/HostsXpert.zip
Unzip it double click on HostsXpert press on Restore MS Hosts File confirm the message by pressing on ok. You must see that under File Handling the first option is set on Make Readonly? Otherwise it will fail. When the changes are applied press on Make Readonly? this will deny any changes to the hosts file.
Without Software
First navigate to "C:\WINDOWS\system32\drivers\etc" ,rightclick on the hosts file, properties read-only should be unchecked press on open select wordpad. Now you will see lots of entries you shouldn't find any entry to security websites, update servers for security programs or any other entries than 127.0.0.1. If not delete them. Save the hosts file when your finished under the same name. Right click once again on the hosts file press on properties and check read-only and press on apply and ok.
Hope this will help.
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