I'm sure to have to look this up again, largely care of:
http://social.technet.microsoft.com/Forums/windowsserver/en-US/504017c7-1a92-415e-b12c-03d44145e1db/how-to-find-which-process-locked-the-log-file?forum=winservergen
Process Explorer (ex Systernals) is good for troubleshooting/ resolving file locks.
You can download it from Microsoft http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx.
Use the icon that resembles a pair of binoculars... or use the Hotkey Ctrl+F(find).
Type the name of the file... it will show you which process is presently running and has locked the file for use.
You can then release the lock should you so wish.
In the case which prompted this quick post the issue was actually the locking level in use with log4net but maybe next time ... ;)