Protecting text boxes in forms

Hello everyone, I was wondering whether anyone out there could please help me?
I am a complete novice concerning MS Access. I have recently managed to
create a database using 2003 and have made some headway but I am stuck on one
particular issue:

When accessing the various forms I would like the text boxes that already
contain data to be locked (this varies from record to record though...) so
that no one can alter them whilst the empty text boxes are editable (and then
locked/protected) once I have entered the relevant data.

Is there a simple way (step by step please as I really haven't got a clue )
of doing this?

Any help would be greatly appreciated,

Sam