Can't update or download security software

For about the past week, none of my anti virus software will update. I get an error message stating that the program cannot connect to the server, on every program. My internet connection is fine, I can go to most sites, however, if I try to go to a site to manually download a virus definition I get a "page cannot be displayed" error message, no matter the site. I've tried to download and install numerous anti virus and malware programs to no avail, either they won't install, or the can't update once they do install. I'm having no other problems other than my browser getting hijacked by "Microsoft Online Security Tool" which I think has something to do with it. I've tried the windows malicious software removal tool, but that didn't find anything after 13 hours of scanning. Can someone please tell what I have to do to fix this OTHER than reformat my drive?