Why don't you use Access' built in user facilities rather than creating your own login/password processing?
Out of interest, how do you stop the user from browsing other's data in the tables of the database?
You can make it so the buttons have code that opens up forms on specific records relating to that user, but as per the above have you thought about how to stop them looking at others?
How about linking the access username with their network login name (assuming they log into the network with a userid) ? You can then make all the queries restricted to querying for the current user.
Here's some info about securing access database generally: