In-order to fix the High CPU usage issue, you can refer the following methods suggested in the Microsoft Community forum.
As suggested here, if you have as I did the Antimalware Service Executable constantly taking 10%+ of CPU usage it seems to be fixed after added C:\Program Files\Windows Defender\MsMpEng.exe to the list of excluded files in the Windows Defender (please see detailed instructions here).
In the following other suggestions …
Check if you can experience the issue in ‘Safe mode with networking’; if safe mode with networking works fine then set the computer to ‘Clean boot’ state to check for software conflicts.
Step 1: Windows Start-up Settings (including safe mode). http://windows.microsoft.com/en-gb/windows-8/windows-startup-settings-including-safe-mode
Step 2: Keep the computer in clean boot and check the issue status. How to perform a clean boot to troubleshoot a problem in Windows Vista, Windows 7, or Windows 8. http://support.microsoft.com/kb/929135
Note: Please go through the section: How to reset the computer to start as usual after troubleshooting with clean boot of the Kb article to boot the computer in normal startup after fixing the issue.
I would also suggest you to run “System Maintenance” to diagnose and fix the issue. It finds and cleans up unused files and shortcuts, and performs maintenance tasks.
Follow these steps:
◾Press “Windows key + S” from keyboard.
◾Type “Troubleshooting” without quotes in the search box and hit enter
◾From “Troubleshooting” windows click “View all” option from the left panel.
◾In the list of options, click on “System maintenance”.
◾Click “Next” and follow the on-screen instructions.
Useful links for possible virus, malware check/removal:
- Malicious Software Removal Tool di Microsoft