Import accounts

Import accounts

Sorry to be a complete newb: I have looked for an answer, honest.
When I try and import accounts from Outlook 2003, which I have saved to a .csv file, I get a message "Please check whether the file is a proper .xls or .csv ...blah blah"
It is a proper .csv, the date format is OK, the top row is names, it's under 5Mg etc.
So tried again, saved as xls and tried to import: same message.