Lot's of things are wrong here, and I'm not even looking at program flow:
1.) Your function prototypes don't match their definitions.
2.) You have two strings named "convertchoice" and "convertwin" in global scope, and it looks like you thought those would be function prototypes(?)
int getComputerChoice(), you're not doing anything. Line 44 does absolutely nothing except waste everybodys time, and then you return zero for no reason.
main() doesn't return anything, although that's probably been magic'd in for you by your compiler.
5.) convertwin, convertchoice, displayResults and determineWinner don't return strings, even though you said they would.
6.) You're not passing parameters by reference, which means that any assignments you make to them do nothing.
7.) Lines 135 and 136 are not inside a function body.