Windows

Yes it is possible You have to configure two lan cards one will go to dsl modem and one will go to yuor local lan You have to use ip addess of the lan card which is going to dsl modem as default getway.You must not forget to enable routing on your lan card whcch is connected to your local lan

The issuse of conflict can be resolved by properly configuring the driver or by changing slot on the motherboard in which you fix the lan card

Microsoft help & support article # 157025 deals with configuration of multihomed systems, those using 2 nics or multiple ip address on one nic.

here's a link: http://support.microsoft.com/default.aspx?scid=kb;en-us;157025

There is another article dealing with multiple nics, # 175767 outlining the expected behavior of mutihomed servers. here's the link: http://support.microsoft.com/default.aspx?scid=kb;EN-US;175767

both will give you the information you want. if the links dont work, visit http://support.microsoft.com and type the article # in the search bar to bring up the article.

Connect the DSL modem to a NAT device (buy a NetGear RP614 for $75) and then plug your computers into the switch ports.
UNLESS, the DSL modem is doing NAT. Some of them do. If the modem does NAT, plug it into a hub/switch and connect everything there.