Embeded Firebird permissions problem under Local System Account

Question

I am creating a NTService Application which use an Embeded Firebird database. When i configure the service run as Administrator account, It works ok. But when i run service as default Local System Account, I get below error.

no permission for read/select access to TABLE XXXX

Why is that ? I belive there is no system tables or etc in Firebird single sql file. And Windows xp dont have linux like file permissions. Please help.

Regards

J