http://msdn.microsoft.com/zh-cn/subscriptions/ms679362(v=vs.85).aspx上比较详细说明，注意两点：
1.You cannot get a valid context for a running thread. Use the SuspendThread function
to suspend the thread before calling GetThreadContext.
2.If you call GetThreadContext for the current thread, the function returns successfully; however, the context returned is not valid.
要先挂起线程，另外不能用GetThreadContext(GetCurrentThread(), &ctx)这种直接获取当前线程的代码