大家看看，求平方根程序在dev-c++通过，却为何在vc6下通不过。
#include <stdio.h>
#include <stdlib.h>
double kf(double x)
{   
    int i;
    double z,y,d;   
    if(x<=0)return x;
    z=x/2.0;i=0;
    while(i<10000)
    {
        i++;
        y=x/z;
        d=y-z;
        if(d<0)d=-d;
        if(d<0.000000000000001)break;
        z=(y+z)/2.0;
    }
    return y;
}
int main(int argc, char *argv[])
{
    double w,x;
    printf("x=");
    scanf("%lf",&x);
    w=kf(x);
    printf("%16.16f\n",w);
    system("pause");   
    return 0;
}

错误信息：
--------------------Configuration: c5 - Win32 Debug--------------------
Compiling...
c5.cpp
C:\Program Files\Microsoft Visual Studio\MyProjects\c5\c5.cpp(10) : warning C4101: 'w' : unreferenced local variable
C:\Program Files\Microsoft Visual Studio\MyProjects\c5\c5.cpp(35) : error C2065: 'system' : undeclared identifier
Error executing cl.exe.

c5.exe - 1 error(s), 1 warning(s)

怪事，重新启动系统（我的为xp系统），关掉360杀毒，再用vc6编译居然通过了，什么都没动，以下是程序及编译信息。
// c5.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <stdio.h>
#include <stdlib.h>
double kf(double x)
{   
    int i;
    double z,y,d;   
    if(x<=0)return x;
    z=x/2.0;i=0;
    while(i<10000)
    {
        i++;
        y=x/z;
        d=y-z;
        if(d<0)d=-d;
        if(d<0.000000000000001)break;
        z=(y+z)/2.0;
    }
    return y;
}
int main(int argc, char *argv[])
{
    double w,x;
    printf("x=");
    scanf("%lf",&x);
    w=kf(x);
    printf("%16.16f\n",w);
    system("pause");   
    return 0;
}
--------------------Configuration: c5 - Win32 Debug--------------------
Linking...

c5.exe - 0 error(s), 0 warning(s)