Here's some code to rename the files

[vba]
Sub RenameFiles()
Dim iLastRow As Long
Dim i As Long
Dim sNew As String
Dim sOld As String

With Worksheets("Sheet1")
iLastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
For i = 2 To iLastRow
If .Cells(i, "C").Value <> "" Then
sOld = .Cells(i, "A").Value & .Cells(i, "B").Value
sNew = .Cells(i, "A").Value & .Cells(i, "C").Value
On Error Resume Next
Name sOld As sNew
On Error GoTo 0
End If
Next i
End With
End Sub
[/vba]