I believe Hyper-V can lock virtual hard disk files for VMs that are "registered" with it, even if the VM isn't running.  I'm not certain and can't check at the moment, but if you were using viBoot for that image and that VM still exists in Hyper-V, try deleting it.  Otherwise, do you have that image mounted for browsing?  Would simply restarting this system be an option to see if that clears up whatever is currently trying to access that file?