I have a group of records set in a form that can be viewed only or modified. 
I need to set an error message when someone tries to modify a record that is
already being modified by another user.  Currently, if this occurs, the user
can make the changes but when they try to save, Access states it cannot as it
is currently being modified.  I need a message to pop up at the start, not
even letting the 2nd user into the record.  How can I do this?

Jennifer wrote:
Use pessimistic record locking.  Tools | Options | Advanced | Default record
locking, "edited record" and mark "open databases using record level locking.
"  This setting applies to *all* tables.  If you only want individual forms
to use pessimistic locking, use either a bound form with the form's record
locks set for "edited record" or else use an ADO recordset with pessimistic
locking (adLockPessimistic) for an unbound form for the record edits.

locked out
security only works on my PC
Locked Out
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AllowBypassKey in Access project
Query Wizard Access
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about readonly of excel file.