unable to save, file in use
 

I have excel 2007 but use compatibility mode because not all user have this
version. we have a small network and we save files on a shared drive. lately
we are unable to save, rename or delete the file; the error message alerts
that the file is in use by another person or program, but we are sure it
isn't. we are forced to save as new file every time. has anybody experienced
the same problem and found its source? How can i see who or which program is
using the file?
Thanks for your help.