Try this:-
Code:
Private Sub Worksheet_Deactivate()
If Range("a1") < True Then
Worksheets("sheet1").Select
MsgBox "You have not completed all the details.", vbCritical, "Incomplete
detail"
End If
End Sub
It wont stop users switching to another workbook, but if you wanted it to,
you could put a similar thing into the workbook module too.