You are running the scannow command in an Administrative command prompt? The command prompt window should not close, but only give some type of message, unless it might be another symptom of your problem.
If you are using the command prompt window, open Task Manager and see if TrustedInstaller is listed on the Services Tab.
If it is, open Services.msc and see if you can start and stop the "Windows Module Installer"? It will probably be not running so don't worry unless you cannot get it to start. The fact you can do Windows updates might rule this out, but won't hurt to check.
If a virus has caused the problem, unless you can somehow identify it there may not be a way to recover without at least a Repair Install. Were you able to boot to a copy of the Microsoft Bootable Anti-virus Elmer suggested?
I have been trying to think of some service that might not be running and causing a problem, but I do not know of one, so I was hoping the File Scan might replace some bad system files.
Otherwise, the things that might be blocking just the Browsers might be an anti-virus, or a firewall, or a previously mentioned virus..