Unlocker used to be a great tool to know which process is using certain file and unlock it. But it’s marked as a malware now.
Process Explorer can also achieve the same result. If you don’t have it, you can download it from https://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
- go to Find->Find Handle or Dll
- type the file name or directory that is used
- Click Search, all processes that are using this file or directory will be listed.