ajax in asp.net

[gautams.mail]

Hi all i have tried to implement ajax
and im trying to use this with webservice.asmx

http_request.open('POST', 'http://..service.asmx/TestConnection',
false);
http_request.setRequestHeader('Content-Type','text/xml');
http_request.send('Id=var');


but this dosent seems to be working . i dont what is wrong with this
code.
please help me.