If you use a workgroup database (e.g. system.mdw) then you must specify the system.mdw in the connection string:

please see http://www.connectionstrings.com/access

example

Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\mydatabase.mdb;Jet OLEDB:System Database=system.mdw;User ID=myUsername;Password=myPassword;

Hope, this helps!