Windows API提供了一个接口GetTickCount，可以获取当前的系统运行时间，通过在开机时运行此程序，即可获得系统的开机时间。下面是MSDN中对GetTickCount函数的说明：

GetTickCount

The GetTickCount function retrieves the number of milliseconds that have elapsed since the system was started. It is limited to the resolution of the system timer. If you need a higher resolution timer, use a

DWORD GetTickCount(VOID)

Parameters

This function has no parameters.

Return Values

The return value is the number of milliseconds that have elapsed since the system was started. 

下面是一个例子：

#include <iostream>
#include <windows.h>
using namespace std;

void main()
{
	DWORD dwCount;
	dwCount=GetTickCount();
	float fCount=dwCount/1000;
	cout<<"您的的开机时间为："<<fCount<<"秒"<<endl;
	getchar();
}

将此程序编译后添加到开机启动项即可。