Easiest output format would be a .csv file. Generate a couple in Excel with the sort of data you want and view them in a text file to see what they look like; then have the script write (or append to) the form data to a text file (with a .csv extension instead of a .txt one) in the same style.
If you want a more Microsoftian file format, you'll probably be mucking around with Excel and its implementation of VB via COM. Either that or heavy reverse engineering of the file format.
Your other question: It's easy to have "two submit actions" - you just need one action script that does both tasks.
---form.php----
1) Get and check form data
2) Write Excel file
3) Do email
