Have a look at the first link I've posted in post #4. Unless you have a very large number of serial numbers (in the tens or hundreds of thousands) you would be better if you replace the textbox with a search combo as it minimizes data entry errors due to the auto-complete feature. The combo would have the bound column the unique identifier (primary key) of your main table and that would be hidden, with a second visible column for the serial.
Open the form unbound or with some criteria to have the fields empty (for the form's record source use "SELECT * from tblMain WHERE True=False;") then in the AfterUpdate event of the combo once you selected the desired serial change the form's recordsource:
Code:
Me.Recordsource=SELECT * from tblMain WHERE ID_PK=" & Me.cboSearchCombo 'assumes the ID is numeric
As for the grey out fields you can disable them by choosing Enabled= False in the data tab of the controls property window.
Cheers,