If you are looking for free file lock software for Windows, first check whether a built-in or free method is enough. Some files only need simple privacy, while payroll sheets, contracts, client documents, USB files, and shared-computer files usually need a more repeatable file-lock workflow.
If each person has a different Windows account, NTFS permissions can limit who can open, edit, or delete selected files. This works best on a PC where accounts are managed carefully.
This is a practical free option for files you store or send occasionally. It is less convenient when you need to open, edit, and re-lock the same file every day.
The hidden attribute can reduce casual visibility, but it is not real file locking. Anyone who knows how to show hidden items can find the files again.
Encrypting File System can protect files for a Windows account on supported editions. It can be useful, but you need to handle account recovery and certificates carefully.
BitLocker is strong for whole-drive protection, especially laptops and external drives. It is less targeted if you only want to lock a few files while leaving the rest of the drive normal.