Popup window in excel 2007

Assuming that you are using data validation for your dropdowns, you can do what you request using a Worksheet_Change macro like:

Private Sub Worksheet_Change(ByVal Target As Range)
Dim cel As Range, targ As Range
Set targ = Range("C2:C10")  'Watch these cells for selection of Independent
Set targ = Intersect(targ, Target)
If targ Is Nothing Then Exit Sub

For Each cel In targ.Cells
    If LCase(cel.Value) = "independent" And cel.EntireRow.Range("H1") = "" Then _
        MsgBox "If you choose Independent in cell " & cel.Address(False, False) & vbLf & _
            "then column H must be filled in as well"
Next
End Sub

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The above code must be installed in the code pane for the worksheet being watched. It won't work at all if installed anywhere else.

As posted, the code watches cells C2:C10 for a user-selection of "Independent". If it occurs, then the code checks whether column H already has a value. If not, then a pop-up message is displayed.
WatchValidationSelectionQ2731298.xlsm

Assuming you are using a Data Validation drop down list for Column C, add this code to the appropriate Sheet module:

Private Sub Worksheet_Change(ByVal Target As Range)
    
    Dim cel As Range
    Dim MsgString As String
    Dim CellCount As Long
    
    If Not Intersect(Me.UsedRange, Me.[c:c], Target) Is Nothing Then
        MsgString = "For the following cells, make sure to populate Column H:" & Chr(10)
        For Each cel In Intersect(Me.UsedRange, Me.[c:c], Target).Cells
            If LCase(cel) = "independent" Then
                MsgString = MsgString & Chr(10) & cel.Address(False, False)
                CellCount = CellCount + 1
            End If
        Next
        If CellCount Then MsgBox MsgString, vbInformation, "Reminder!"
    End If
    
End Sub

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these are very close.  what if you have another column U that is dependent on the text "Termination" that will require column V and column W to be filled with dates.