Public Function numtoword(numstr As Variant) As String
' The best data type to feed in is
' Decimal, but it is up to you

Dim tempstr As String
Dim newstr As String
numstr = CDec(numstr)

If numstr = 0 Then
  numtoword = "zero "
  Exit Function
End If

If numstr > 10 ^ 24 Then
  numtoword = "Too big"
  Exit Function
End If

If numstr >= 10 ^ 12 Then
  newstr = numtoword(Int(numstr / 10 ^ 12))
  numstr = ((numstr / 10 ^ 12) - _
  Int(numstr / 10 ^ 12)) * 10 ^ 12
  If numstr = 0 Then
    tempstr = tempstr & newstr & "billion "
  Else
    tempstr = tempstr & newstr & "billion, "
  End If
End If

If numstr >= 10 ^ 6 Then
  newstr = numtoword(Int(numstr / 10 ^ 6))
  numstr = ((numstr / 10 ^ 6) - _
  Int(numstr / 10 ^ 6)) * 10 ^ 6
  If numstr = 0 Then
    tempstr = tempstr & newstr & "million "
  Else
    tempstr = tempstr & newstr & "million, "
  End If
End If

If numstr >= 10 ^ 3 Then
  newstr = numtoword(Int(numstr / 10 ^ 3))
  numstr = ((numstr / 10 ^ 3) - _
  Int(numstr / 10 ^ 3)) * 10 ^ 3
  If numstr = 0 Then
    tempstr = tempstr & newstr & "thousand "
  Else
    tempstr = tempstr & newstr & "thousand, "
  End If
End If

If numstr >= 10 ^ 2 Then
  newstr = numtoword(Int(numstr / 10 ^ 2))
  numstr = ((numstr / 10 ^ 2) - _
  Int(numstr / 10 ^ 2)) * 10 ^ 2
  If numstr = 0 Then
    tempstr = tempstr & newstr & "hundred "
  Else
    tempstr = tempstr & newstr & "hundred and "
  End If
End If

If numstr >= 20 Then
  Select Case Int(numstr / 10)
  Case 2
  tempstr = tempstr & "twenty "
  Case 3
  tempstr = tempstr & "thirty "
  Case 4
  tempstr = tempstr & "forty "
  Case 5
  tempstr = tempstr & "fifty "
  Case 6
  tempstr = tempstr & "sixty "
  Case 7
  tempstr = tempstr & "seventy "
  Case 8
  tempstr = tempstr & "eighty "
  Case 9
  tempstr = tempstr & "ninety "
  End Select
  numstr = ((numstr / 10) - _
  Int(numstr / 10)) * 10
End If

If numstr > 0 Then
  Select Case numstr
  Case 1
  tempstr = tempstr & "one "
  Case 2
  tempstr = tempstr & "two "
  Case 3
  tempstr = tempstr & "three "
  Case 4
  tempstr = tempstr & "four "
  Case 5
  tempstr = tempstr & "five "
  Case 6
  tempstr = tempstr & "six "
  Case 7
  tempstr = tempstr & "seven "
  Case 8
  tempstr = tempstr & "eight "
  Case 9
  tempstr = tempstr & "nine "
  Case 10
  tempstr = tempstr & "ten "
  Case 11
  tempstr = tempstr & "eleven "
  Case 12
  tempstr = tempstr & "twelve "
  Case 13
  tempstr = tempstr & "thirteen "
  Case 14
  tempstr = tempstr & "fourteen "
  Case 15
  tempstr = tempstr & "fifteen "
  Case 16
  tempstr = tempstr & "sixteen "
  Case 17
  tempstr = tempstr & "seventeen "
  Case 18
  tempstr = tempstr & "eighteen "
  Case 19
  tempstr = tempstr & "nineteen "
  End Select
  numstr = ((numstr / 10) - Int(numstr / 10)) * 10
End If
numtoword = tempstr
End Function

Thanks to an anonymous person from Cheats World for this alternative, shorter function:

Dim Number2s(2 To 9) As String
Dim Number1s(1 To 19) As String
Dim Steps(1 To 4) As String

Public Function NumToWord(Numstring As Variant) As String
If Steps(1) = "" Then initstrings
Dim Tempstring As String
Dim Newstring As String
Dim What As Double

If Numstring = 0 Then
  NumToWord = "zero"
  Exit Function
End If

If Numstring > (10 ^ 24) Then _
  NumToWord = "Number is too big"
  Exit Function
End If

Counter = 0
Do
  Counter = Counter + 1
  If Counter > 1 Then
    What = 10 ^ (12 \ ((Counter - 1) * 2))
  Else
    What = 10 ^ 12
  End If

  If Numstring >= What Then
  Newstring = NumToWord(Fix(Numstring / What))
  Numstring = ((Numstring / What) - Fix(Numstring / What)) * What
  If Numstring = 0 Then
    Tempstring = Tempstring & Newstring & Steps(Counter) & " "
  Else
    If Counter = 4 Then
    Tempstring = Tempstring & Newstring & _
    Steps(Counter) & " and ": GoToSkipit
    Tempstring = Tempstring & Newstring & _
    Steps(Counter) & ", "
    Skipit:
  End If
End If

Loop Until Counter = 4

If Numstring >= 20 Then _
Tempstring = Tempstring & _
Number2s(Fix(Numstring / 10)) & " "

If Numstring >= 10 And Numstring < 20 Then _
Tempstring = Tempstring & Number1s(CInt(Numstring)) _
& " ": GoTo Skip2

Numstring = ((Numstring / 10) - Fix(Numstring / 10)) * 10

If Numstring > 0 Then Tempstring = Tempstring & _
Number1s(CInt(Numstring)) & " "
Skip2:

NumToWord = Tempstring

End Function

Public Sub initstrings()
Steps(1) = "billion"
Steps(2) = "million"
Steps(3) = "thousand"
Steps(4) = "hundred"
Number1s(1) = "one"
Number1s(2) = "two"
Number1s(3) = "three"
Number1s(4) = "four"
Number1s(5) = "five"
Number1s(6) = "six"
Number1s(7) = "seven"
Number1s(8) = "eight"
Number1s(9) = "nine"
Number1s(10) = "ten"
Number1s(11) = "eleven"
Number1s(12) = "twelve"
Number1s(13) = "thirteen"
Number1s(14) = "fourteen"
Number1s(15) = "fifteen"
Number1s(16) = "sixteen"
Number1s(17) = "seventeen"
Number1s(18) = "eighteen"
Number1s(19) = "nineteen"
Number2s(2) = "twenty"
Number2s(3) = "thirty"
Number2s(4) = "forty"
Number2s(5) = "fifty"
Number2s(6) = "sixty"
Number2s(7) = "seventy"
Number2s(8) = "eighty"
Number2s(9) = "ninety"

End Sub