Router and Lan configuration problem

Trying to setup small home network (2 PCs) to share Internet connection; connect through AOL, so acquired Thompson (Alcatel)SpeedTouch 510 wired router. Cabled OK. Every time I run Setup for SpeedTouch it alters the LAN settings (both machines set to ‘Optain Auto IP’); sometimes I can connect to Internet through one PC(Win2000) and access local machines; other times I can access Internet through XP Pro machine, and other local machines; cannot connect through both, or even one after disconnecting from the other. Not sure if this is possible, but if one is disconnected (from Internet, not from LAN), why not? I run the setup on one machine after the other; normally the first one works, but not the second… First router, perplexed…(if this is the wrong board, please redirect…)