Using Automation you can create an instance of Excel and work with it:
' This needs a reference to Excel (here for Office 2003):
' Tools --> References --> Microsoft Excel 11.0 Object Library.
Dim xlapp As Excel.Application
Set xlapp = New Excel.Application
' It's also possible to use Late Binding.
' (This does not need a reference to Excel but is less efficient).
' Dim xlApp As Object
' xlapp = CreateObject("Excel.Application")
' Useful when you're debugging.
.Visible = True
' Open the Excel file.
.Workbooks.Open "C:\Custom Error Codes.xls"
' Select the proper sheet.
' Select the upper-left cell (A1).
' Write something into it.
.Selection = "Something"
' Save the file.
' Then close it.
' Close Excel. !!! This is very important !!!
' If you do not property close the instance of Excel it will remain
' locked in memory with no visible interface (only Task Manager to kill it).
' Clean up
Set xlapp = Nothing