Mark Russinovich at www.sysinternals.com has released freeware utility Handle which lets you answer that question - which program has a particular file or directory open? Useful to Windows NT admin.

Handle For Windows NT is targetted at searching for open file references, so if you do not specify any command-line parameters it will list the values of all the handles in the system that refer to open files and the names of the files.

New Riders has good NT texts.