How to stop multiple instances of Excel 2003 opening

Thanks CC - sounds as if it could work.
But if in an Explorer window, I click Tools Folder options, I do not get a File types tab. Thus cannot follow your advice. Any ideas?
I am using Win7/64, not that that seems to make any difference - I have never seen a File types tab in any version of Windows.

Also, there doesn't seem to be anything wrong with file associations - double clicking an xls file always opens it.

Ran the regserver routine again using the full path, but Excel couldn't find stdole32.tlb, so it looks as if I may need to reinstall or repair Excel. Will try that next.

LATER - did a repair of Office, made no difference. Maybe a full uninstall, including all registry entries, and reinstall might fix it. But can't be bothered. For the time being, will continue to drag and drop the file onto the open instance. A bit of a pain, but manageable. Hope someone has a better idea.