If you're on a Windows box, fire up a DOS prompt and run "tracert
www.google.com". That should give you a basic idea of where the bottlenecks are. If on a Mac, the equivalent is "traceroute".
You should probably also compare browsing speeds between a web address and an IP, say,
www.google.com and 173.194.71.103. If there is a significant difference, your ISP is most likely having DNS trouble.
If that is the case, you should be able to use another DNS server such as Google's 8.8.8.8 or Open DNS's 208.67.222.222. Usually, this involves editing a modem/router configuration page but I don't know the details of your setup.