3 parts to this script
=====================================================
PART 1
====================================================
<SCRIPT>
<!--

var opStack = new Array(4)
opStack[0]  = 0
opStack[1]  = ''
opStack[2]  = ''
opStack[3]  = ''
function start()
{
document.PAD.SUM.value= "0"
}
function KeyinNum()
  {
  if (opStack[0] >= 2) { opStack[0] = 3 } else { opStack[0] = 1 }
  opStack[opStack[0]] = '' + document.PAD.SUM.value
  }
function display ()
  {
  var sum  = parseFloat(opStack[1])
  var huge = Math.pow(10,14)
  sum      = Math.round (sum * huge) / huge
  document.PAD.SUM.value = '' + sum
  }
function resetNum ()
  {
  opStack[0] = 0
  opStack[1] = 0
  document.PAD.CAL.value = ''
  document.PAD.SUM.value = '0'
  }
function entry (x)
  {
  if (opStack[0] == -1) { opStack[0] = 1; opStack[1] = ''}
  if (opStack[0] == 0)  { opStack[0] = 1; opStack[1] = ''}
  if (opStack[0] == 2)  { opStack[0] = 3; opStack[3] = ''}
  var result = result = opStack[opStack[0]]
  if (result == '0')    { result = ''                                   }
  //---------------------------------------------------------------------
  if (x>='1' && x<='9') { result = '' + result + x                      }
  else if    (x == 'P') { result = '' + Math.PI                         }
  else if    (x == '0') { if (result != '') result = '' + result  + '0' }
  else if    (x == 'B') { if (result != '') result = result.substring(0, result.length - 1) }
  else if    (x == '.') {
    if (result != '')
      { if (result.indexOf(".") == -1) result += "." }
    else
      { result = '0.'                                }
    }
  //-----------------------------------------------------------------
  if (result =='') result = '0'
  opStack[opStack[0]] = result
  document.PAD.SUM.value = result
  }
function neg()
  {
  if (opStack[0] != 2  && opStack[0] != 0)
    {
    opStack[0] = Math.abs(opStack[0])
    var result = opStack[opStack[0]]
    if (result != '0' && result != '')
      {
      if (result.charAt(0) == '-')
        { result = result.substring(1, result.length) }
      else
        { result = '-' + result                       }
      opStack[opStack[0]] = result
      document.PAD.SUM.value = result
      }
    }
  }
function calc1 (x)
  {
  var opFlag = opStack[0]
  if (opFlag == -1 || opFlag == 1)
    {
    count(x)
    }
  else if (opFlag == 3)
    {
    opStack[1] = '' + eval(opStack[1] + opStack[2] + opStack[3])
    count(x)
    }
  }
function count (x)
  {
  if      (x == 'Si') { opStack[1] = '' + Math.sin (opStack[1])    }
  else if (x == 'aS') { opStack[1] = '' + Math.asin(opStack[1])    }
  else if (x == 'Co') { opStack[1] = '' + Math.cos (opStack[1])    }
  else if (x == 'aC') { opStack[1] = '' + Math.acos(opStack[1])    }
  else if (x == 'Ta') { opStack[1] = '' + Math.tan (opStack[1])    }
  else if (x == 'aT') { opStack[1] = '' + Math.atan(opStack[1])    }
  else if (x == '¡Ô') { opStack[1] = '' + Math.pow (opStack[1],.5) }
  else if (x == '^2') { opStack[1] = '' + Math.pow (opStack[1], 2) }
  else if (x == '^3') { opStack[1] = '' + Math.pow (opStack[1], 3) }
  else if (x == '^4') { opStack[1] = '' + Math.pow (opStack[1], 4) }
  else if (x == 'AB') { opStack[1] = '' + Math.abs (opStack[1])    }
  else if (x == '¡×') { }
  document.PAD.CAL.value = ''
  opStack[0] = -1
  display()
  document.PAD.SUM.focus()
  document.PAD.SUM.select()
  }
function calc2 (x)
  {
  var opFlag = opStack[0]
  if (opFlag != 2)
    {
    if (opFlag == 3)
      {
      opStack[1]=''+eval(opStack[1]+opStack[2]+opStack[3])
      display()
      }
    opStack[0] = 2
    opStack[2] = x
    document.PAD.CAL.value = x
    document.PAD.SUM.focus()
    document.PAD.SUM.select()
    }
  }
function fra()
{
var i
var j = 1
var k = document.PAD.SUM.value
if (k >= 70)
{opStack[1] = 0; document.PAD.SUM.value = "---Error---"; return}
for (i = 1; i <= k; i++)
{
j = j * i
}
opStack[1] = document.PAD.SUM.value = j
  document.PAD.SUM.focus()
  document.PAD.SUM.select()
}
function TurnOff()
{
var ask = confirm("This will close the window, continue?")
if (ask == true) {window.close()}
else {return}
}
//-->
</SCRIPT>
==============================================================
PART 2
===========================================================
<BODY ONLOAD="start()">

===============================================================
PART 3 - put this where you want the calculator to show
================================================================
<!-- --><FORM NAME="PAD">
<TABLE BORDER>
<TR><TD COLSPAN=8>
<INPUT NAME="CAL" TYPE="TEXT" SIZE=1 VALUE="">
Result:
<INPUT NAME="SUM" TYPE="TEXT" SIZE=36 VALUE="" onChange="KeyinNum()">
<TR>
<TD><INPUT TYPE="BUTTON" VALUE="C"      onClick="resetNum()"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="OFF"      onClick="TurnOff()" WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="Square"      onClick="calc1('^2')" WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="X !"      onClick="fra()"  WIDTH=100>
<TR>
<TD><INPUT TYPE="BUTTON" VALUE="1"        onClick="entry('1')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="2"        onClick="entry('2')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="3"        onClick="entry('3')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="+"        onClick="calc2('+')" WIDTH=100>
<TR>
<TD><INPUT TYPE="BUTTON" VALUE="4"        onClick="entry('4')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="5"        onClick="entry('5')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="6"        onClick="entry('6')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="-"        onClick="calc2('-')"  WIDTH=100>
<TR>
<TD><INPUT TYPE="BUTTON" VALUE="7"        onClick="entry('7')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="8"        onClick="entry('8')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="9"        onClick="entry('9')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="X"        onClick="calc2('*')"  WIDTH=100>
<TR>
<TD><INPUT TYPE="BUTTON" VALUE="0"        onClick="entry('0')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="."        onClick="entry('.')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="="        onClick="calc1('=')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="÷"        onClick="calc2('/')"  WIDTH=100>
<TR>
<TD><INPUT TYPE="BUTTON" VALUE="Backspace"        onClick="entry('B')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="SQRT"        onClick="calc1('¡Ô')"  WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="Pi"        onClick="entry('P')"   WIDTH=100>
<TD><INPUT TYPE="BUTTON" VALUE="+/-"        onClick="neg()"           WIDTH=100>
</TABLE>
</FORM><!-- -->