not opening files

open their outlook file. It gives an error message stating, “The file H:\Outlook\Email Folder_newold(1).pst is in use and cannot be accessed. Close any application that is using this file, and then try again. You might need to restart your computer.I closed and reopened Outlook, restarted the computer, and closed and tried to reopened that file and still get the same error message. The anti-virus isn't running a scan and the backups don't start until around 7:00pm, so I don't know what could be locking up a .pst file besides Outlook. So far I don't think Windows have a tool that would tell you what process is locking a file and Task Manager doesn't help