Some 100baseTx compatible devices work only at 10baseT

I have a network that connects 11 computers, one copier, one router.

Hardware:
Network Switch - Linksys EtherFast 10/100. (EZXS16W)
Router - 2Wire HomePortal 1070
10 Computers running Windows XP
1 Computer Windows Me
Copier - Canon IR C3220

There are a few computers that I can leave the speed of the network card or inboard network at "Auto" or "100 full duplex". The network on these computers is fast if I transfer files ONLY between these computers.

Those other computers I have to manually configue the speed to "10 half duplex". Otherwise they don't get the right IP address, in other words they don't connect to the network.

The copier has all options but only "10 half duplex" works.

When I configure all those "10 half duplex" devices to "automatic" or "100 full duplex", the lights on the switch come on just fine. They somehow just don't communicate.

Could it be that the Router (2Wire HomePortal) is messing up and comunicates only at "10 half", thus the devices don't get an IP address if they don't communicate with the Router? Why would there be a few computers that work at "100 full duplex" then?

Can anyone help me please?