Macro to copy from one workbook to a different workbook

I need to copy data from one workbook to another workbook. The problem is that the receiving workbook will always have a different name. Here is my macro:

Sub Add_Units_Worksheet()
'
' Add_Units_Worksheet Macro
'
    Sheets.Add After:=Sheets(Sheets.Count)
    Sheets("Sheet2").Select
    Sheets("Sheet2").Name = "Units"
    Windows("Test File.xls").Activate
    Range("A1").Select
    ActiveSheet.Paste
    Cells.Select
    Cells.EntireColumn.AutoFit
    Range("A1").Select

End Sub

I need the line Windows("Test File.xls").Activate to allow the user to choose the file to be activated, preferably without having to type in a file name. This file will already be open.

Any help anyone can provide would be much appreciated.

Thanks.
Mikki