Process Explorer

Task Manager tells you something is busy; Process Explorer tells you what and why. Written by Mark Russinovich and published by Microsoft as part of Sysinternals, it shows every process with its owning account, and its lower pane lists the handles and DLLs each one has open or loaded. The search box is the reason to install it: type a filename and it shows which process has that file locked. It is a single executable with no installer, so it can live on a USB stick or run straight from Microsoft's Sysinternals Live share. Current builds list Windows 11 and Windows Server 2019 or later as supported.