well, yer not giving us a lot to go on.
It could be your DNS configuration in the TCP/IP control panel. It could be your ISP is having problems.
I would double check all your settings in TCP/IP and go from there. If it's a new System folder you may want to make sure you have all the updates from apple installed. And if it's not a new system folder, mb it's time to do a clean install.
-Al