Okay then, make use of the Characters mehtod ..

Sub MakeWordBold()
    Dim strSearch As String, searchRng As Range, i As Long, cel As Range
    Set searchRng = Range("A1:A10")
    strSearch = InputBox("Please enter the text to make bold:", "Bold Text")
    If strSearch = "" Then Exit Sub
    For Each cel In searchRng
        With cel
            .Font.Bold = False
            For i = 1 To Len(.Text) - Len(strSearch) Step 1
                If Mid(.Text, i, Len(strSearch)) = strSearch Then
                    .Characters(i, Len(strSearch)).Font.Bold = True
                End If
            Next i
        End With
    Next cel
End Sub