Some malware hides while the OS is running. Use Microsoft Defender Offline or a portable tool like Emsisoft Emergency Kit from a USB drive. This scans the system before the malware has a chance to "wake up". How to Run a Full Scan Using Windows Security
: Always update your virus database first. Scanning with old definitions is like a sheriff looking for outlaws with a 10-year-old "Wanted" poster. virus scanner
A "Deep Scan" (often called a Full Scan) goes beyond common folders to examine every bit of data on your storage drives. : Some malware hides while the OS is running