Will it generate a flood if I will send a UDP message indefinitely to a network/LAN using winsock? If someone's going to do such kind of program, will it slow down the network?   

Nah, it won't.  UDP packets are generally less than 2,000 bytes in size.  Assuming you send 10 UDP packets a second at that size, that's 20,000 bytes, or about 20 kbps.  Your slowest LAN would be 10 mbps (10,000 kbps)  So you'd only be 'using' about 0.2% of your network's capacity.  Most people have a 100mbps network (100,000 kbps) so the issue would be even less of a concern.

How about TCP, Will it slow down the network?

How about TCP, Will it slow down the network?