Since your target is a spreadsheet on Windows, it would make sense to write
directly to the spreadsheet. You can use either the xlsReadWrite package,
Description: Read and write Excelfiles natively (v97-2003/BIFF8)
or RExcel, which allows you to imbed R functions inside Excel cells.
Look at the main RExcel/rcom site
http://sunsite.univie.ac.at/rcom/
and consider joining the mailing list there.