Are you saying web sites load in one OS but not the other? If so, it's unlikely your ISP is to blame.
In order to load a web page, the browser must first resolve the site name into an IP address. That requires a working DNS resolver in the OS, which means the OS must be configured to use one or more working DNS servers. An incorrect DNS server entry can cause considerable delays as the OS waits for the request to time out.
You can test the DNS resolver with the host, dig or nslookup commands. nslookup www.somesite.com should return the IP address of the web server. If it doesn't or if it takes more than a second or two, check your DNS settings.
Once the name has been resolved, a TCP connection must be established to the web server. Since the server is not on your local network, the IP packet must be routed through a gateway. Misconfigured IP parameters (wrong netmask, more than one default gateway) can cause failures and/or delays.
traceroute www.somesite.com will show you the path an IP packet has to traverse to reach the server.
You can use telnet or nc (netcat) to simulate a web browser connection to TCP port 80. telnet www.somesite.com 80 should return "Connected to www.somesite.com". An error message means no TCP connection could be established.
A possible source of TCP connection delays is a non-functional dual stack configuration. As you may know, the next-generation IP protocol, IPv6, is being deployed on the Internet. Not all ISPs support IPv6, though. If your OS is configured to use both IPv4 and IPv6 (dual stack) and the site in question supports IPv6, the web browser may attempt to connect using IPv6 rather than IPv4. If your ISP is still stuck in the dark ages doesn't yet support IPv6, the browser will have to wait for the IPv6 connection to time out before falling back to IPv4.
ping6 ipv6.google.com should tell you if your OS supports IPv6. A reply means all is fine, "timeout" or "destination unreachable" means you have no IPv6 route and should probably just disable IPv6 altogether.
Finally, you may also want to check the proxy settings in the browser, although an incorrectly configured proxy setting would usually result in total loss of web connectivity.