I've scanned with Malwarebytes, AVG and Kaspersky and while they found some malware and deleted it, the issue still persists. I've tried changing the mouse settings, still nothing. I've disabled the laptop mouse and plugged in a USB one, still the same thing.
Also, my PC seems a tad slower and some things can hang like, the taskbar along with the start menu or opened programmes top bar, until I Ctrl-Alt-Del and click anywhere on the screen.
All my browsers are affected by this too, probably because they think both mouse buttons are being pressed.