This works with excel 97:
print the data as CSV; send it to the browser with Content-type: application/msexcel

Of course it may show up as plain text in the browser unless the user has the mime-type set up. You could alternately send the content as a MIME-encoded email, with:

MIME-Version: 1.0
Content-type: application/msexcel; name="yourFile.xls"; boundary=42
[download]