Clients PCs locking up

I have NT server 4.0 Sp 6 with mix of win 98 and win me. Connected by coax cable..
At end of day users power off their PC. In the morning when they power up some of them hang when the login .. Some times some of the have to reboot/ Some hang up when they try to access the database on the server..
The server alsways stay on..
What could be causing this .. Thanks all