codice:
Dim CarSostituto As String * 1
Dim CarDaSostituire As String * 1

Dim Stringa As String
Dim Cont As Long

CarDaSostituire = "*"
CarSostituto = " "
Stringa = "Stringa*di*prova"
Cont = 1

Cont = InStr(1, Stringa, CarDaSostituire, vbTextCompare)
Do Until Cont = 0
  Mid$(Stringa, Cont, 1) = CarSostituto
  Cont = InStr(Cont, Stringa, CarDaSostituire, vbTextCompare)
Loop