Try Tools | Macro | Security and set it to low. If you get emails, be sure you know what you're opening. Also, disable the preview in Outlook, otherwise, a spreadsheet could be opened automatically if the email is html, it has an object reference to excel and it's the top item that's selected.
You can also create yourself a Digital Certificate by running the self-cert program that comes with Office:
Run the Office 2000 setup program again.
On the Select Features screen in the setup program, expand Office Tools.
Select Digital Signature for VBA projects, click the arrow next to your selection, and then select Run from My Computer.
In Windows Explorer, locate and then double-click the file SelfCert.exe in C:Program FilesMicrosoft OfficeOffice.
Note Because a digital certificate that you create yourself isn’t issued by a formal certification authority, macro projects that are signed by using such a certificate are referred to as self-signed projects. Depending on how your organization uses the digital-signature features in Microsoft Office, you might be prevented from using such a certificate, and other users might not be able to run self-signed macros for security reasons. Learn about digital signatures.
For more information about creating digital certificates, see Help in Visual Basic for Applications.
This should allow you to keep your security settings at medium, and as long as you tell the computer on which you are using the spreadsheet that you are a trusted source, it will only ask you the first time you use the file if you wish to enable macros. Everytime after that it should just load automatically.