My laptop last night decided to slow right down and CPU was running at 100%?

I only had one browser window open, on one tab. I have run a virus scan, used Ccleaner, done a defrag, and when i came to work this morning I left it in the process of doing a Malwarebytes full scan

Does anyone know why this would happen, as you can imagine when its at 100% you can't do anything on the bloody thing without it taking 100 years!