I was confused about format ACCDB, therefore to let others to know:
Definition: The ACCDB database file format was introduced with the release of Microsoft Access 2007 to replace the older MDB file format. The use of ACCDB format allows users to leverage enhanced functionality in Access, including the use of multivalued fields and strong cryptography.
So far I tried to find out the solution, but I have observed that you will only be able to convert this format by using MS 2007.
Open this file in MS 2007, you can also download a trial version from microsoft site:
And save the file as previous MS Access format. To work with it in MS Access 2003.