Hey, could I ask you to check the system tray and see if you have a process called "LocalServiceNoNetworkFirewall" with a sub-point called "BFE"?
I've had an issue which sounds the same. It has a high usage of "Disk" and some CPU usage. It appeared to block internet access to the point where it took extremely long, just to go to the simplest sites. Also, when it really acted up, it made my computer lag like crazy. Just opening Start was a serious challange.
I've just uninstalled my Premium Avast and it seemed to have solved the issue completely.
LocalServiceNoNetworkFirewall appears to be a Microsoft program, which has ties to Windows Defender. So I suspect that Avast and Windows Defender are having relationship issues at the moment.
In any case, does any of what I just said sound familiar? Are you able to find the "LocalServiceNoNetworkFirewall" process, or do I need to start up a new thread? :p
UPDATE: I've re-installed Avast Premium and everything works fine now. So a simple, complete, re-installation fixed my issue, apparently.