Well, you can simply use the Access password so that anyone opening it up needs to provide the password. If you want to use another method, you will (if you use an ACCDB/ACCDE/ACCDR file format) need to create your own information and then use code or macros to check each time someone does something whether they have entered the correct information.
The drawbacks are that Access security is not all that tight. The database password in 2007 is better than previous versions but I think that there are now hacking tools out there for that. But if you use your own, it is even less secure - really. Because you will have to lock down various things which are actually fairly easy to unlock if someone knows how.
If you really need security, then perhaps using SQL Server Express as a backend along with Access as the frontend is your way to go.