If when accessing a database inside a web service you encounter the following error when invoking your dataset:

The Microsoft Jet database engine cannot open the file '(unknown)'. It is already opened exclusively by another user, or you need permission to view its data.

This is caused because your database is stored in a folder or location where IIS does not have permission to access it. USB drives, portable hard drives, and floppy disks (i.e. any removable storage device) by default has no permissions set on it and can therefore bypass this problem. Simply moving your DB to one of these devices and reestablishing your connection to them will remove the access error.