You should describe more about what you want to do, but typically if you have something that needs to run in the background and does not require direct user interaction, then a service often makes sense.
You can use Remoting to connect a front-end to your service as needed if you'd like.
Yes, use a service for this kind of operation, but don't use filesystem watcher. If you poll for files in your service, dont use the Timer class either.
Do check to make sure the file is completed writing and is no longer locked before trying to move it.
Its trivial to poll for file changes (syntax may be off), and eliminates much of the extra programming associated with file system watcher events.
While True 'or your exit condition'
Dim _files() as FileInfo = Directory.GetFiles(yourTargetDirectory)
For Each _file as FileInfo In _files
If _file.LastModifiedDate < DateTime.Now.AddMinutes(1) Then
'move your files'
End If
Next
End While
Using a Windows service to wrap FileSystemWatcher is perfectly fine for what you need.
FSW is the right tool for the job (native code for filesystem watching is a bear to get right), and a service is the right mechanism to deploy it given you need 'always on' operation.
The service credentials will be independent of logged-in user too, which may be useful to you.