The combination of extreme slowness , browser redirection to unknown sites , outdated antivirus , and blocked access to security settings strongly indicates a malware infection localized to that device (especially since other workstations and network diagnostics are normal). Quentin Docter lists browser redirection as a common security symptom ("Your browser is redirected to websites") and stresses that if you experience even one of these security symptoms, you should "immediately rectify the security issue or quarantine the system ." In malware removal best practices, the first operational containment step after verifying symptoms is to isolate
/quarantine the infected system to prevent spread, credential theft, or further compromise. Disconnecting the device from the network accomplishes this containment so investigation (logs, scans, offline tools, safe mode, etc.) can be done without risking other systems. Clearing cache (B) is too narrow and doesn't address blocked security settings/outdated AV. Firewall checks (A) and raising settings (D) don't contain the threat and may be prevented by the malware itself. Therefore, the next step is to disconnect and investigate .