S/W to poll if a file date/time has changed

Hello all.
I have a need to be able to interrogate if a single file has changed (via date/time/size), and if so then do something (i.e. copy it to a backup folder, etc). The routine should always be running when Windows starts.

Is there a windows-based applet (shareware) that can do this?

Any help is appreciated.
Thanks!
Walter