I have seen many issues with "file in use".

What are you trying to do with the file that is in use?
Copy from, copy to, read, change etc...
does it require exclusive use or can you share it?

If it is a dbf file, and your accessing data, make sure whatever routines access it are letting it go (closing) at some point.
Issuing a tablupdate() or a USE (to close) can sometimes help.

More specifics may be required to help any further...