Process and files

You should read this articles first.

Know which process has locked a file
http://windowsxp.mvps.org/processlock.htm

Download Links.

Process Explorer
http://www.sysinternals.com/Utilities/ProcessExplorer.html

Windows Server 2003 Resource Kit Tools
http://www.microsoft.com/downloads/details.aspx?FamilyID=9D467A69-57FF-4AE7-96EE-B18C4790CFFD&displaylang=en

WhoLockMe Explorer
http://www.dr-hoiby.com/WhoLockMe/index.php

I have used Process Explorer to find which process is locking which file.

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