codebehind method and ajax

Hi.

i have a webservice which returns a dataset from database, the webservice works fine, i want to call my codebehind method on every keypress from javascipt, which inturn call this service and this codebehind method is used to fill a gridviewcontrol from the returned dataset.

so how to call my codebehind method using ajax everytime when a user on clientside presses a key???.

note: i donot want to use PageMethods techinique.

Any one can Help ???

thx in advance

Mansoor.