How can I modify an Excel worksheet or Office document from a build?
 

Attached is a sample (taken from the 'Editing a Workbook' sample in http://support.microsoft.com/default.aspx?kbid=260410).

Some things to be aware of when working with those samples:

1) Remove the ' As <Excel.Application>' from variable declarations (everything is a variant in VBScript).
2) VBScript doesn't support named parameters (i.e. xlApp.Workbooks.Open FileName:="c:\My Documents\Book1.xls"); pass all required parameters in the order required without naming.
3) Use the actual numeric value of constants defined in the Excel object model, since these constants aren't available from script.

See the Microsoft Excel Visual Basic Reference in the Excel help for details on 2 and 3.