How can I hook Windows functions in C/C++?

Take a look at Detours, it's perfect for this sort of stuff.

For system-wide hooking, read this article from MSDN.

First, create a DLL which handles hooking the functions. This example below hooks the socket send and receive functions.

#include <windows.h>
#include <detours.h>

#pragma comment( lib, "Ws2_32.lib" )
#pragma comment( lib, "detours.lib" )
#pragma comment( lib, "detoured.lib" )

int ( WINAPI *Real_Send )( SOCKET s, const char *buf, int len, int flags ) = send;
int ( WINAPI *Real_Recv )( SOCKET s, char *buf, int len, int flags ) = recv;  
int WINAPI Mine_Send( SOCKET s, const char* buf, int len, int flags );
int WINAPI Mine_Recv( SOCKET s, char *buf, int len, int flags );

int WINAPI Mine_Send( SOCKET s, const char *buf, int len, int flags ) {
    // .. do stuff ..

    return Real_Send( s, buf, len, flags );
}

int WINAPI Mine_Recv( SOCKET s, char *buf, int len, int flags ) {
    // .. do stuff ..

    return Real_Recv( s, buf, len, flags );
}

BOOL WINAPI DllMain( HINSTANCE, DWORD dwReason, LPVOID ) {
    switch ( dwReason ) {
        case DLL_PROCESS_ATTACH:       
            DetourTransactionBegin();
            DetourUpdateThread( GetCurrentThread() );
            DetourAttach( &(PVOID &)Real_Send, Mine_Send );
            DetourAttach( &(PVOID &)Real_Recv, Mine_Recv );
            DetourTransactionCommit();
            break;

        case DLL_PROCESS_DETACH:
            DetourTransactionBegin();
            DetourUpdateThread( GetCurrentThread() );
            DetourDetach( &(PVOID &)Real_Send, Mine_Send );
            DetourDetach( &(PVOID &)Real_Recv, Mine_Recv );
            DetourTransactionCommit(); 
        break;
    }

    return TRUE;
}

Then, create a program to inject the DLL into the target application.

#include <cstdio>
#include <windows.h>
#include <tlhelp32.h>

void EnableDebugPriv() {
    HANDLE hToken;
    LUID luid;
    TOKEN_PRIVILEGES tkp;

    OpenProcessToken( GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, &hToken );

    LookupPrivilegeValue( NULL, SE_DEBUG_NAME, &luid );

    tkp.PrivilegeCount = 1;
    tkp.Privileges[0].Luid = luid;
    tkp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;

    AdjustTokenPrivileges( hToken, false, &tkp, sizeof( tkp ), NULL, NULL );

    CloseHandle( hToken ); 
}

int main( int, char *[] ) {
    PROCESSENTRY32 entry;
    entry.dwSize = sizeof( PROCESSENTRY32 );

    HANDLE snapshot = CreateToolhelp32Snapshot( TH32CS_SNAPPROCESS, NULL );

    if ( Process32First( snapshot, &entry ) == TRUE ) {
        while ( Process32Next( snapshot, &entry ) == TRUE ) {
            if ( stricmp( entry.szExeFile, "target.exe" ) == 0 ) {
                EnableDebugPriv();

                char dirPath[MAX_PATH];
                char fullPath[MAX_PATH];

                GetCurrentDirectory( MAX_PATH, dirPath );

                sprintf_s( fullPath, MAX_PATH, "%s\\DllToInject.dll", dirPath );

                HANDLE hProcess = OpenProcess( PROCESS_CREATE_THREAD | PROCESS_VM_OPERATION | PROCESS_VM_WRITE, FALSE, entry.th32ProcessID );
                LPVOID libAddr = (LPVOID)GetProcAddress( GetModuleHandle( "kernel32.dll" ), "LoadLibraryA" );
                LPVOID llParam = (LPVOID)VirtualAllocEx( hProcess, NULL, strlen( fullPath ), MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE );

                WriteProcessMemory( hProcess, llParam, fullPath, strlen( fullPath ), NULL );
                CreateRemoteThread( hProcess, NULL, NULL, (LPTHREAD_START_ROUTINE)libAddr, llParam, NULL, NULL );
                CloseHandle( hProcess );
            }
        }
    }

    CloseHandle( snapshot );

    return 0;
}

This should be more than enough to get you started!