pcID is a text field. Be default on a new pc it will have no value or computer name.
If IsNull(DLookup("[pcID]", "tblUserSecurityAutoLogin")) Then
DoCmd.SetWarnings False
DoCmd.RunSQL "UPDATE tblUserSecurityAutoLogin SET tblUserSecurityAutoLogin.pcID'" = "[txtComputerName].Value"
DoCmd.SetWarnings True
End If