can't connect

Can some help? This is my home network. I have a robotics 4 port wireless router.I have cable internet connected that I share with 4 computers. 1 wired computer 1 wireless with a access point 1 laptop wireless and 1 wired that I setup for working on computers. I do this for a hobby. this is my problem. every once in a while, when I connect a computer to work on it. most of the time I just have to plug the ethernet plug in and I can access the internet. sometimes I have to reset everything (router, modem). I have this dell now and it was infested with syware and adaware. when I first started, I was able to connect. now I can't. my ip addresses are usually 192.168.123..... but on this computer it always comes up 169.254.115.48 with the subnet 255.255.0.0. even when I manually set the ip. I can ping the router, and all the computers that are connected, but I can't ping outside of that.I've noticed when that ip comes up, I can't get online. I tried the release and renew but that don't work. I connectthis computer directly to the modem and it still stays the same.even when I reset the modem. I have no problems with the other computers. does anyone have any suggestions? Thanks. also I am running xp sp1 on a dell 4550 dimension.