I came looking for an answer to the same sort of problem except with version 7.

While I am not certain of all the details it appears that once a file is opened by an application in the virtual machine the lock persists so long as that application is open.  Time is *NOT* a factor, I have seen locks persist for days.

What's really nasty is that when the file was opened by Windows itself this still applies--but note that Windows never closes.  The lock persists until the VM is shut down/rebooted.