Any way to determine what process is "using" a given file?

A lot of times when I go to rename or delete a certain file, especially media files of a few hundred megs or more, I find I cannot do so for a LONG time, because I get the message "Cannot delete [file] because it is being used by another program ".

Sometimes this goes on for over 5 minutes.

I have tried disabling my resident virus scanner, when this happens, and yet I still get the error.

There are other times, in the past, I've had this message persist until I reboot (various file types for various reasons).

It would be nice to be able to tell what any given file is being accessed by.