The process cannot access the file because it is being used by another process. :-P
Seriously, my painful recollections are that many Windows programs take out exclusive locks on anything that they open, and those locks block all other operations. So if anyone else wants to open your file for anything, they can cause huge problems.
My first guess would be to look for users who might be opening a file in some kind of editor (or Excel, or whatever else). My second guess would be to ask what happens if your program is trying to move a file that some automated job (eg a virus scan) happens to be looking at.
This information is based on things that used to be a PITA for me quite a few years ago. The default behaviour could well have changed and I have no relevant recent experience.
Cheers,
Ben