It should be in C:/Windows/System32/calc.exe
if it isn't, then here you go:
http://shoutbox.menthix.net/attachment.php?pid=549070
Simply place it back into the system 32 folder, and it should work, then just make a shortcut in the Accessories folder in All Programs
.
I think the keyboard button will always be set to work as a calculator, so by replacing this, it should definitely work...
Your system32 folder might also be hidden, to unhide this, go to Control Panel > Folder Options > View > Show hidden files and folders.
Attachment: 
