if the cpu is stuck at 50% usage when theres nothing running other than your av. then your looking for malware... and b4 you say it. yes malware can infect a system thats protected by an anti-virus as there isnt 1 thats 100 percent.
star scanning with decent 1s like avira and malwarebytes. i can almost guarantee it finds something especially if you have avg, avast or worse norton installed.
open task manager and see which process is taking up the most cpu time and thats likely to be your infection, if its ccc or mom thats doing it then you need to flush your gfx drivers with driversweeper or similar.and install freshly downloaded 1s.