Sim Chee Siang,

You can use your uC as an stand alone unit and don't need a PC to input/output the data in your calculator. You can use a LDC and a Keypad to do I/O

Let me tell you there is also another programming language -Basic- you can use BASCOM for 8051(free version limited to 2KB) it can handle arithmetic functions and a LCD, there are code examples for a keypad.

If you decide to use 8051 we can help you otherwise you need to go to a PIC forum.

Regards