WiFi Problem

Many ISP's have a set computername for the one to be added to their network, Make sure that the router has that ID. Also, check that the DHACp range is set correctly on the router. Check that you are not blocking any ports that you need to access the internet, and that proxy settings are correct for the ISP (many new networks use no proxy settings, including autodetect. None should be checked in those cases).

well, since you are a pro you have undoubtably pinged everything and it must work and just neglected to mention. plus you have already checked out your dns and gateway settings and successfully pinged them but again figured we would assume all this.
so have you tried ipconfig /release /renew?
something is 'wrong' with tcpip still. what address does ipconfig say bad computer is at? maybe no good address so try release renew if dhcp. otherwise, sounds like improper gateway or dns settings on the bad machine. are they both set to use dhcp? if yes, are they both set for router as gateway?
if you can ping your isp's dns servers, then you are all set except dns is not working. either configure the bad machine manually for the dns servers specified by your isp (this is cheating but will work) or see if changing the gateway settings on bad box to internet router address works.

Make sure your connecting to your router and not someone else's router. Also, check to see if you have any bridges going. And make sure your LAN settings are set to dectect automatically and they are not set to high settings to blockeverything. If you anything like NAV\Mcafee\ZoneAlarm they might be blocking you from getting outside.

If you can successfully ping other computer on the network, I'd try to ping a web-site outside, ex: Ping www.google.com.. If you are able to ping outside then your IP setting should be correct. If this is the case then most likely it's a problem with Internet Explorer.

REPAIR INTERNET EXPLORER... Windows XP..

http://support.microsoft.com/default.aspx?scid=kb;EN-US;q318378

Cheers
Digital_DNA

Are you actually connecting to your router and not another one in the area?

Win 98SE