The Silverlight MIME type may need to be defined within the IIS server. To check that, go to the Control Panel, click Administrative Tools, and start the Internet Information Services (IIS) Manager.

• Windows Vista: Choose the "Default Web Site" and run MIME Types manager. Make sure the .XAP extension is in the list, if not, add it as mime type application/x-silverlight-app.
• Windows XP: Right-click the "Default Web Site" and choose "Properties". Go to the HTTP Headers tab and Click File Types... Make sure the .XAP extension is in the list, if not, add it as mime type application/x-silverlight-app.