In the task manager-->processes tab-->if you click on the 'CPU' column, it will sort the processes by CPU usage (2 click = highest use on top).
If you've got a system process (other than the 'system idle process') hogging the CPU, that's a good place to look for a problem.
To access safe mode with networking, restart the computer. When it's starting up, keep tapping F8. You should enter the safe mode menu, which looks like this:http://www.geckoandfly.com/wp-content/uploads/2009/11/windows_safe_mode_xp_2000_f8.jpg
Arrow down to 'safe mode with networking' and hit enter. Click the messages to proceed into safe mode. It will look very similar to normal windows. Then update malwarebytes, and do your scan.