How can I insert alphabets values only in a textbox ?

Java-Script for Accepting Alphabets only.

function IsAlpha(txtvalue)
{
var numaric = txtvalue.value ;
for(var j=0; j 64 && hh<91) || (hh > 96 && hh<123) || (hh == 32) || (hh==46))
{
}
else
{
txtvalue.focus();
txtvalue.value = "";
alert('Please Insert [a-z] only number are not allowed!');
return false;
}
}
return true;
}