I would also recommend LibreOffice. However, I tend to use gnumeric,
which comes from the gnome world and is much smaller. It can read .xls
and .xlsx, but probably doesn't handle some of the more esoteric
features. It does everything I actually need, including graphs.
I have no idea if gnumeric on windows is easy to deal with. It works
fine on Unix-like systems (Linux, *BSD, and runs on Mac with X).