ajax - PagerequestManagerParserErrorException

I am new to Ajax and trying to  integrate Ajax in my application(.NET framework 2.0)...I am facing an issue which i couldn't resolve...Here is the error message

System.WebForms.PageRequestManagerParserErrorException:The message received from the server could not be parsed. Common causes for this error are when the response is modified by the calls to Response.Write(),response filters, HttpModules or the server trace is enabled..

I did find lot of forums with this Error Message but none of them were helpful....My web application doesn't have Response.Write, response.filters but i do have a HttpModule(which stores the cookie information), I modifed the code to handle the requests from ajax differently using the code snipette

if (HttpContext.Current.Request.Headers["x-microsoftajax"] == null)

but still doesn't work....Any suggestions??