By "Crystal Report Viewer", are you talking about a web application?  If so, there is an access rights issue.

Web applications run under a machine user ID that is local to the web server where they're running.  Because this is a local ID, it does not have any access to network resources such as the folder where your Excel file is stored.  If this application is on your network and not on a public web server, you'll need to find a way to have the website run under a network user ID.  If the application is a public-facing web server, this is a security risk.  Also, if it's a public-facing web server, it should be in a DMZ with a firewall between it an the network, so you won't be able to get to any of the network resources from the server even with a network ID.