OK, if you can open it in Gnumeric, then in Gnumeric, do a Save As, make sure you give it a different name, and save it as an XLS file, (or Excel format). Then I bet you'll be able to open the new copy in Calc.
Entirely correct.
I don't quite know how you ended up with an XML file though!
Er, by assuming Excel 2003 was a shareable option... (Shows as a text/xml option if that answers Mark's question.) If I need to use Calc again, I shall be more careful.
Bev.