Client side code to accept only numbers in textbox.

For the textbox add this handler:
TextBox1.Attributes.Add("onkeydown", "JavaScript:return CheckifNumeric();");

Add this script:
function CheckifNumeric()
{
var retValue = false;
var mykey= (window.event.which) ? window.event.which : window.event.keyCode;

if ( ((mykey>= 48) && (mykey<= 57)) || (mykey== 8) || (mykey== 13) )

retValue = true;

if ( window.event.returnValue )

window.event.returnValue = retValue ;

return retValue ;
}