Lots of people have reported problems, here on AB, where they can browse the internet for a short while but then nothing will load in their browser after that. Disconnecting and reconnecting always provides a fix (albeit a very temporary one) for the problem.
I've no idea why this occurs but, in every case, the solution seems to be to specify the addresses of your ISP's domain name servers, rather than having them automatically assigned. (If that sounds rather technical, don't panic. It's simply a matter of typing in a couple of sets of numbers).
Right-click on the icon for your connection to the internet. Select 'Properties', then 'Networking' and 'TCP/IP Settings'. Click the button alongside 'Specify name server addresses'.
Alongside 'Primary DNS' change the string of zeros to 194.168.4.100. Alongside 'Secondary DNS' enter 194.168.8.100. Leave the other lines unchanged.
(NB: Those numeric addresses apply to accounts with Virgin Media which, from your post, I assume applies to you. If you're actually connecting via another ISP, go to their support pages for the relevant DNS addresses. Alternatively, enter 208.67.220.220 and 208.67.222.222 respectively. Those addresses are for open DNS servers, which can be used by anyone).
Then click 'OK' and (hopefully) the problem should be fixed.
(NB also: My instructions refer to Windows ME. The way you get to your TCP/IP settings might be slightly different in XP and Vista but the method will still work. If you see 'Apply' displayed, click it before clicking 'OK').
Chris