Open infopath file from webpart

I have a Infopath Library in a SharePoint site and have few files in it. Now I wast to add a web part in the home page with a textbox and a button. I wast user to enter a form name in the textbox and when click the button the form should open in the diffrent windows. Right now I have a btn_Search_Click method for the button and have this code:

Response.Redirect("http://site/_layouts/FormServer.aspx?XmlLocation=/Lib/" + txt_Search.Text + "&Source=http://site/Lib/Forms/AllItems.aspx&DefaultItemOpen=1");

when I click this button i get this error : The following file is not a valid InfoPath form: /Lib/filename. Can someone help?