If you can go to some web-sites but not most, it's probably your ISP's name-server.
My ISP has the same problem frequently. Luckily, I have a win2k server box and I just turn it on when I want to surf and my ISP is having probs.
You might want to google for a reliable public name server and plug it's IP addy into your routers secondary dns, if you have one.
If you connect without a router, maybe hand-set (via ipconfig utility) an IP for a name-server.
hope this helps