Is the other directory where the excel files are stored mapped as a virtual directory within your app? If not, you can't link to them like this. You can only hyperlink to content which is directly accessible from the web. It may work when you try it locally, but as soon as you deploy the app to a server, and try to access the files, you will find that it fails, as the lonks will be pointing to local resources rather than valid urls.
If you want to store the files outside of your web app virtual directory structure, you could use linkbuttons instead, and then on postback read the contents of the selected file and write it to the response stream using the Response.TransmitFile method.