Well, first, you need some constructors for your Computer class if you want to pass arguments to them. Without the proper constructors, the following line will cause errors:

Computer compaq(50,0.5,8), toshiba('A'), gateway('C');

Second, there is no show method defined in Computer, you define display, but it doesn't take zero arguments.

Finally, return ; should be return 0;.
Code:
#include<iostream>
using namespace std;

class Computer
{
public:
  Computer ( char a );
  Computer ( int b,double c,int d ) :
    cpuSpeed ( b ), hardDrive ( c ), memory ( d )
  {}
  void show();
private:
  int cpuSpeed, memory;
  double hardDrive;
};

Computer::Computer ( char a )
{
  switch ( a ) {
  case 'A':
    cpuSpeed = 75;
    hardDrive = 1;
    memory = 16;
    break;
  case 'B':
    cpuSpeed = 100;
    hardDrive = 1.5;
    memory = 16;
    break;
  case 'C':
    cpuSpeed = 133;
    hardDrive = 2;
    memory = 32;
  }
}

void Computer::show()
{
  cout<<"CPU: "<< cpuSpeed
    <<"\nHD: "<< hardDrive
    <<"\nMem: "<< memory
    <<'\n'<<endl;
}

int main()
{
  Computer compaq ( 50, 0.5, 8 ), toshiba ( 'A' ), gateway ( 'C' );

  compaq.show();
  toshiba.show();
  gateway.show();

  return 0;
}
>These are easy assignments your instructor is giving you.
Maybe for you, but these kind of problems tend to give obscure error messages.

-Prelude