Very weird problem with the client

I had to refresh my MSI with i5-4200H 2.80, 8GB RAM due to a malware that created a lot problems. The real problem began when the client started to act very funky. Everytime I log in, the client flickers. A new LOL icon keeps reappearing in the task bar replacing the one before. Once I join the game, I do not have any problem playing the game except I can't alt-tab back in the game once I alt-tab out in FULL-SCREEN mode. Both borderless and windows modes do not have problem with alt-tab, but I am experiencing a significant amount of FPS drop (180 in full screen to 20-40 in borderless and window). I also tried to start the client in Windows 7, but the problem remained the same. What's interesting is the fact that while the game is running, LOL client in the task manager says it's not responding.

Has anyone experienced this problem? or a possible solution to this?