OpenOfficedotorg Calc works opens xlsx files and works with them just fine, only it can't save as xlsx as well - you'd need to save as xls or xml (Excel 2003 XML). But generally there should be no problem saving as xls/.xml unless you are using some very unique Excel 2007 features that aren't available in older versions.
"Gnome Office tells to support XLSX but what about their compatibiltiy (""supports: write: incomplete"") or Windows support (their main target is Linux) i cannot say, did not tried.
Microsoft Works version 9 (also known as Microsoft Works 2007) also is claimed to support reading and writing XLSX files. Though its copy, if not for some leftovers, can hardly be purchased, as the product was obsoleted and replaced by Office Starters (provided to computer OEMs, not to users themselves). However such a product did existed.
