Data at the root level is invalid. Line 1, position 1 in MOSS,WSS

Today when I working on wss 3.0 / MOSS site suddenly I got an error saying , "Data at the root level is invalid. Line 1, position " . Then I come to know I have open one html file from virtual directory in SharePoint Designer, SharePoint designer automatically geneates some folder inside this directory which is the cause of an above error. so when I deleted this folder named "_vti_cnf" then site works fine..