Start by finding out the primary and secondary DNS addresses used by your ISP. (They're simply sets of numbers, like 208.67.220.220). If you're with one of the UK's major ISPs, they're probably listed here:
http://www.idl.uk.com/email/isp_settings.htm
If not, they'll be on the support pages of your ISP's website. Alternatively, you can use these addresses, which should work for all ISPs:
Primary DNS = 208.67.220.220
Secondary DNS= 208.67.222.222
Then disconnect from the internet and click the following. (NB: These are the instructions for XP. Other version of Windows might vary slightly but you're still trying to get to the 'TCP/IP' settings):
Start > Control Panel > Network and Internet Connections > Network Connections.
Right-click on the icon for your internet connection, and select 'Properties'.
Select the 'Networking' tab. Click 'Internet Protocol (TCP/IP)'. Click 'Properties'.
Click to select 'Use the following DNS server addresses'. Enter the two sets of numbers. Click 'OK'. (As far as I'm aware, there's not an 'Apply' button in there, but if there is, click it before clicking 'OK').
Reconnect to the internet and cross your fingers. When everything works properly, send me a large drink ;-)
Chris