Disable TextBox on Click CheckBox Using Java Script

Try This:-

function DisableTxt(txt,chk)
{
if(document.getElementById(chk).checked==true)
{
document.getElementById(txt).value=0;
document.getElementById(txt).disabled=true;
}
else
{
document.getElementById(txt).disabled=false;
}
}

Numeric Validation Using Java Script

Try This:-

function Numeric(Id)
{
If(IsNaN(Id.Value))
{
alert('only Numeric');
}
}