Honestly, I think you will really find it difficult to fit the code in the 1K program memory of a 16F84. C can produce much larger code than (well-written) assembler and probably won't fit.
You would be very well advised to learn assembler for smaller MCU's like the 16F84. It really isn't that hard once you start, use other people's code as examples. There are plenty of tutorials and advice on the net, and plently of people to help you when you get stuck. You will gain a much better understanding of how these things actually work, making you able to use them and program them better.
I assume that you only want a simple +-/x calculator, a scientific is completely out of reach in a 16F84. I'm not sure you can even fit a simple one a 16F84, given that you must:
Scan and decypher keyboard
Store the keypreses
Perform floating-point arithmatic up to (8? 9?) digit accuracy
Store the interim and final results
Store the displayed result
Talk to an LCD
Just my opinions... Best of luck whatever you decide to do.
Cheers,
FoxyRick.