There is a Knowledgebase that discusses this very issue, and an example for LabVIEW 6 of how to do it. You can go to www.ni.com and search for "+activex +excel +save" (without the quotes), or use the link below:
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/702b1bd0123a15128625695e00564983