My computer has been infected by viruses, malware and security threats (opens in new tab)

My computer is clearly infected with some kind of virus or malware, as I keep getting persistent pop-up ads even when my browser is closed, unfamiliar toolbars and extensions have appeared without my permission, and my system performance has drastically slowed down with the hard drive constantly running at full capacity for no apparent reason. I've already run full scans with Windows Defender and Malwarebytes, but they either find nothing or detect threats that they fail to completely remove,...