Javascript not working in IE on different server

I found a javascript function that gets called onkeypress that checks what key is entered, returns true if it's a control character, number, or decimal point, and false if it's anything else.  This is for monetary fields, etc...to allow only numbers to be entered into that field.  This works perfectly on a remote server I use for web hosting when accessed through both IE and Firefox.  But when running locally on my Apache web server, it works fine in Firefox, but doesn't function in IE (the exact same IE browser on the same machine).  It doesn't give any errors, it just doesn't do its job.  It seems to be reading the keys properly, because if I put alert statements into each of the cases, it alerts from the right spot (number! in the number spot in the code and letter! in the letter spot of the code, that sort of thing).  Does anyone have any ideas?  Why would the server it's running on make a difference for javascript?

Here's the function:
function numbersonly(myfield, e)
{
var key;
var keychar;
var dec = 1;

if (window.event)
   key = window.event.keyCode;
else if (e)
   key = e.which;
else
   return true;
keychar = String.fromCharCode(key);

// control keys
if ((key==null) || (key==0) || (key==8) ||
    (key==9) || (key==13) || (key==27) )
   return true;

// numbers
else if ((("0123456789").indexOf(keychar) > -1))
   return true;

// decimal point jump
else if (dec && (keychar == "."))
   {
   return true;
   }
else
   return false;
}
-----------------------------------
(I'm not sure why myfield is there, since it doesn't seem to be used, but whatever.)  Anyway, it's called from within an input field in the form like this:

<input type ="text" size ="16" onChange ="validateCCnum();" onKeyPress="return numbersonly(this, event);" maxlength ="16" name ="ccnumber">

I've tried taking out all other function calls (the validateCCnum, etc...) and that made no difference.  All other js functions are working properly.