The first thing to do is ensure all your security software is fully updated.
Go to 'My Computer' and right click. Select 'Properties' and then click the 'System Restore' tab. Tick the box that says 'Turn off System Restore on all drives'. Click 'Apply' and 'OK'.
Restart your PC in Safe Mode and run full scans with Avast, Malwarebytes and Defender. This should clean out the threats.
If they are still present, it could be that a rogue security software package has installed. Search Add/Remove for anything you don't recognise and either ask here or investigate online.
Whilst online you could run one of the free checks such as HouseCall for extra confidence.
When you are satisfied the PC is clean go to 'My Computer' and restart 'System Restore' by unchecking the tick box, clicking 'Apply' and 'OK'.