Look in Task Manager under Processes tab
Click on CPU column header so that services get sorted based upon % usage
What's at the top ?
I have frequently seen this occur when ya have an AV subscription that has expired ..... and had it go away after uninstalling.
I'd suggest downloaded 30 day free trial of Bit Defender or Zone Alarm internet security suites .... Disconnect from internet, uninstall ay current AV software, make sure to run registry cleaner to remove all AV entries after uninstall, then reboot.
Install one of those alternates and see if it changes things ... if not, ya can uninstall new one and put original software back.