Excel can only hold 15 significant digits in a number, so the end will just be truncated with zeros - very annoying.

From memory if you open a csv by clicking on it and your pc is configured to open csv's in excel it just works out what is best (and often isnt)

I would have thought that the CSV file would have the correct long number recorded correctly? If so can you import into excel and use the import wizard then in step 3 you can select Text.

If you can upload an example csv i can give it a test

Edit:
Whoops - i hadnt seen joeu2004's post. At least we agreed!