what's the most reliable way to check a pc for malware?

Random redirects are very often a browser extension rather than something on the PC. Go through your extensions and remove anything you don't recognise or didn't install on purpose, then check the browser's search engine and startup pages haven't been changed. After that, run the Defender Offline scan, not the quick one. Windows Security > Virus & threat protection > Scan options > Microsoft Defender Offline scan. It reboots and scans before Windows loads, so it catches things that hide while Windows is running. A second opinion from Malwarebytes free doesn't hurt either, just uninstall it after if you don't want two products.