This is a nasty trick, but it works in recent version of excel.
You just save the html table as a normal html file, then just rename it to xls. If your application is a web application use response.setHeader("Content-Disposition", "attachment; filename=order.xls").
Recent version of excel support Microsoft Office XML Formats, so if the previous solution doesn't work, you may want to try to generate the xml from the html table.
See XML Spreadsheet Reference.
What I do, is to generate the xml from excel in the xml format, then hack into it. You can implement also advanced features like automatic filters, and cool layouts. I used a template engine to generate the xml (Freemarker, it's Java stuff, but I'm sure there are template engines also for .NET)
.xlsextension. If the file has that extension it should open in Excel