Windows 98 lock up?

I am in an office with 30 new Dell Dimension pIII’s. (1) Three of them lock up over night every night. I have taken them off the network, taken the screen savers off, redone TCP/IP. The mouse still works, but the desktop is locked. (2)When the PC is rebooted, I have trouble connecting to the internet and email. (Through the network) 10 minutes pass and the connection is fine.
All three PC’s are on different circuits. I have moved them from one port to another switched them with other PC’s, etc. The problem is within the PC’s, but what I’m not sure.