Excel 2007 to Oracle connect issues

Firstly apologies if this is not the right place for this question but I thought I'd start here rather than microsoft forums (I've already searches existing forums for a solution).

When I refresh my excel 2007 pivot table to my Oracle Table Excel gives error: ORA-01005 null password given logon denied. If I click ok to the error message it prompts for my username and password which i enter and the pivot refreshes fine. I do not contain my password in the connection string for security reasons but assume there must be a way for it to prompt for password prior to attempting to connect? Interestingly when i check the database audit logs it does not show any failed connection attempt.