I read your post but there are not enough clues.
I will note that many pests hijack the Windows Sockets. Or some install some PPPoE software and forget to remove it.
The short and wild guess here is...
1. Scans with Housecall and Ewido.
2. Run WINSOCKFIXXP
3. Run IEFIX
On a different house the same problem but with two laptops. They both pick up the wireless signal but only one has Internet access.
They are defferent laptops. It doesn't matter if I connect the RJ45 from the DSL to the laptops, one works and the other one doesn't.
They both have a firewall (on).