using a switch vs using a hub

Win2K Server, clients Win9x and XP. Some network cards are 10mbps, others are 10/100mbps. TCP/IP configured on all clients. Using a 10 mbps hub everything was fine, changed to a 10/100mbps switch, now no one can log on, can’t see anyone from server, can’t ping anyone. All ports on the switch are lit, also lights on network cards lit. Why is this?