I have written  a small program to your problem have a look at my code
below

Sub om()
Dim a As String
Dim temp As String

a = Range("a1")
strlen1 = Len(a)
i = 1
While (strlen1 <> 0)
temp = Mid(a, i, 3)
If (temp = "FOX") Then
gotit = "Yes"
pos = i
End If
i = i + 1
strlen1 = strlen1 - 1
Wend
If (gotit = "Yes") Then
MsgBox ("Fox does exist at " & pos & " character to " & pos + 3)
Else
MsgBox ("Fox does not exist")
End If
End Sub

I hope this will be useful to you.