This code assumes no headings in columns C and D, and that the data starts in row 1 and there are not blank rows within, and puts the resulting combinations in row E:
Code:
Sub combos()
Dim ws As Worksheet
Dim c As Integer
Dim d As Integer
Dim e As Long
Dim cNumb As Integer
Dim dNumb As Integer
Dim eNumb As Long
Application.ScreenUpdating = False
Set ws = ActiveSheet
With ws
cNumb = .Range("c1").End(xlDown).Row
dNumb = .Range("d1").End(xlDown).Row
For c = 1 To cNumb
For d = 1 To dNumb
eNumb = eNumb + 1
.Range("e" & eNumb).Value = .Range("c" & c).Value & "\" & .Range("d" & d).Value
Next d
Next c
End With
End Sub