How big is the set of files you want to monitor? Because in most cases (and with a proper filter) 'Process Monitor' from SysInternals can show you your (set of) monitored file(s), which process has doing the modifying and when the modifying did occurred. It doesn't prevent any modification though.
Perhaps auditing software is more of a fit to your request. A free solution from Netwrix
In combination with proper ACL settings on the set of files you wish to keep an eye on, auditing software might be the only thing you need. If I remember correctly, 4wd mentioned recently that 'SetACL Studio
' could now be used for free. More precisely, the creator of the software provides a license code with which you can register SetACL Studio gratis. An alternative piece of freeware, that also happens to be portable as well, could be: NTFS Permissions Tools
(is from China, but has an English interface).
* edit: added NTFS Permissions Tools link.
* edit2: added SetACL link