Really weird 100% CPU usage issue by no particular process

Hi there! As you will see, English is not my first language (by far )
Some days ago I noticed after a couple of minutes I start mi PC, my CPU usage goes to a constant 100% , so I checked for viruses and such but MSE gave me nothing and neither Malwarebytes.

Here's the weird thing, I checked the Task Manager to see witch process was causing this and it's always a different process (Sometimes MsMpEng.exe, sometimes dwm.exe and even taskmgr.exe itself) so I started killing processes like a maniac leaving just the few essential ones but nothing changed.
I also stopped all the services with a delayed start, since the problem starts after a couple of minutes but the problem was still there.

Any other idea will be really appreciated.

P.S. My system restore is disabled.

Work through these Troubleshooting Steps for Windows 7 starting with establishing a Clean Boot.