I think I had this problem a long time ago with Ajax2.0 or 3.5. Which version are you using? And also there are some add ons in IE those are required to run the ajax. Also please mention the browser and test it on different browser like IE, Firfox and chorome and see the result.
these are the answers of ur questions.
1) version is 3.0.20820.0.
2) in FF page is postback when i do not input value in one of the required textbox and press enter or press submit button and in IE & chrome it does nothing.
please give me other solutions. i stucked here since last two days.i don't know why it is not working.