I had a scenario where I had a remote user on a Wind 7 Pro SP 1 64 bit machine connecting to my SBS 2011 box and getting an error about Excel files being in use by 'another user.' I turned off the auto-preview in Windows Explorer to no avail. I looked through this page also to no avail:
http://blogs.technet.com/b/the_microsoft_excel_support_team_blog/archive/2012/05/14/the-definitive-locked-file-post.aspx
The eventual solution was the registry entry change from the client machine described here:
http://support.microsoft.com/kb/942146
