Validating and processing asp.net textbox

Hi,
I have a currency conversion application that needs some Javascript to do some validation on the 'onkeypress' event on a textbox (txtAmount) so that:=
if the validation passes (ie a number is entered) then the application populates a textbox by multiplying the contents of the entire textbox by an exchange rate (ie txtAmount contains 1234, each number entered is validated correctly then the entire contents is * by an exchange rate) 
or
the validation fails and a message is sent to the user to warn him that a non-numebrik keystroke has been entered and also, deletes the erroneous keystroke that has been entered (ie 1234X will produce an error and the 'X' will be deleted from txtAmount)
I do not know Java, so any help would be much appreciated, there is an article on the site that appears to do the trick, but I cannot down load it.
Thanks
Paul