FPS Drops Fullscreen

May 6, 2017
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So I've had my PC for months now and I noticed whenever I try to play a game in fullscreen I always get FPS drops/spikes. For an example we'll take Overwatch. before this whole thing happened I could play overwatch at 2560x1080 ultra settings with 90+ FPS but now for some reason the game just gets a lag spike about every 10 seconds causing the game to go down to about 10-20 FPS about 2-3 seconds. This whole thing happened when I downloaded a weird file for some reason and I got my PC infected (How dumb can I be right?) so I gave the PC to someone that fixed it, so it shouldnt be caused by mallware right? I've also checked to see if my CPU was throttling, but it wasn't. I hope all of you can help me with this problem and specs are down below.

GPU: Msi GTX 1070 Gaming X
CPU: i5-4460 3.20Ghz
Ram: 16gb Corsair Vengeance
Motherboard: Msi Z97 Gaming 5
CPU Cooler: Deepcool Captain 360