lag problems

hi!

My notebook is a new HP Pavilion 15 p020ng, I bought it in September this year. Everything was fine till nowadays. My operation system was Windows 7 64x before I installed Windows 8.1 64x. Problems begun while I had Win 7. I noticed that my notebook became so slow, booting the system took more time than usually, opening folders and browsers became so slow, but the main point was when I lagged in games. My fps was around 150-160 in League of Legends with normal graphic settings, and now I have 30 and lagging horribly. After these I installed Windows 8.1, I hoped it will manage the problem. With the new operation system everything was fine since I installed it (for a week), but now my notebook became so slow again, I have fps lag in games again. Ofcourse I rebooted it so many times, checked which programmes are running, how many memories do they take, but there is nothing special.  So I have the same problem with both operation system on my notebook. All the settings are set to more perfromance, but it doesnt help. Do you have any idea?

Thanks!