While hosting silver light application in IIS .IIS should allow the request of XAP file type. In IIS 7, it default it configured with XAP file types. But in IIS 6 and IIS 5 we need to add a file type that maps the .xap extension. Below are the steps to add files in iis.
1. Select your website in IIS–>right click–>Properties –Properties window will open.
2. In Properties windows–>select HTTP Headers Tab->In MIME MAP–> click on File Types.
3. In File Types–>Click New Type–> Type below text in textbox
Associated extension: .xap
Content type (MIME): application/x-silverlight-app
4 Click on OK–>OK–>Apply