It could be a DNS ( = 'domain name server') problem. Here's a quick test:
Instead of typing in an address like www.google.com, try putting this in your address bar (and, of course, clicking 'Go'):
72.14.205.103
If you still don't see any web page, it's not a DNS problem and everything that follows is pointless. However, if you find yourself looking at Google's web page (which I strongly suspect that you will), it's definitely a DNS problem.
All websites have a 'real' numeric address (such as the one I've given you for Google). When you type in a 'www' address, your request is routed to your ISP's domain name server, which converts it to the numeric address. Sometimes your PC can fail to access the DNS correctly, leaving you with no access to the web.
Domain name servers also have numeric addresses. To fix the problem, you need to know the addresses of the two domain name servers used by your ISP. (There's a 'primary' and a 'secondary' server, which share the load between them). The information will be buried deep somewhere among the support pages on your ISP's website. (Or you phone them and ask them). However, if you're with one of the UK's principal ISPs you can probably get the information easiest by simply looking here:
http://www.idl.uk.com/email/isp_settings.htm
Now go to Start > Settings > Dial-up Networking. (That's the route in Windows Millennium, which I'm using. It might be slightly different in other versions of Windows). Right-click on the icon for your internet connection. Select 'Properties'. Click the 'Networking' tab. Click 'TCP/IP' settings. Click on the 'radio button' next to 'Specify name server addresses'. Enter the primary and secondary addresses. (Leave the other lines showing zeros). Click 'OK' and 'OK' again.
Chris