Some "internet nasties" modify the Hosts file so as to block access to sites which might remove the infection, e.g. AntiVirus and antimalware sites.
The Hosts file is a plain text file, but with no extension, which can be opened in Notepad. It is located in:
Open the file and check that there isn't an entry something like:
127.0.0.1 www microsoft com
and with similar entries starting 127.0.0.1 for the domains of your AntiVirus sites. If there are, delete the lines, save the file and exit. (Dots were removed from the domain to prevent the forum software messing it up with a "click here".)