You could use:
document.onkeyup = function(e){
e= window.event || e;
if(67==e.keyCode) openCalculator();
}
I think it should work better than Accesskey.
EDIT: Just thought of this, you need to change:
window.open("calculator.asp","Calculator1",'resizable=yes, scrollbars=yes,Titlebar=Calculator,toolbar=false,status=yes,menubar=false,width=450,height=450');
To
window.open("calculator.asp","_blank",'resizable=yes, scrollbars=yes,Titlebar=Calculator,toolbar=false,status=yes,menubar=false,width=450,height=450');
If the second param is named (set to something other than _blank), it won't open in a new window everytime, it will open in the one named Calculator1, so once it has one with the name, it won't open new windows anymore.