"cannot copy 'filename' : Access is denied .The source file may be in use"

i have a windows2003 with AD on a compaq server with RAID5
i have made a hierarchy of directories on one volume for different devisions
of company to archive files.i have set all the share permissions and
security and they all work.
problem is sometimes users can not access the files which they normaly can.
they get an error like:

"cannot copy 'filename' : Access is denied .The source file may be in use"

when they try to copy the files from server to their local computers.
i am sure the file is not open.problem sometimes goes away with re copying
the file.
it has happened on different directories and with diffrent
users.Administrator user is able to copy this files but again the files are
corrupt from time to time.and do not be opened with the application.
any help or idea woould be severly appreciated.