This type of problem has been reported loads of times here. I used to have it myself but I found a solution that works. (I haven't got a clue why the problem happens and I don't know why my solution works. It just does!)
I've posted my way of fixing this lots of times and, with only one exception, it always seems to have worked. (The only guy who found it didn't help may have additional problems with his router).
So, for what it's worth, here's my suggestion:
First, find out the primary and secondary DNS addresses used by your ISP. (If you're not technically-minded, don't let that panic you. They're just sets of numbers, such as 195.92.195.94). If you're with one of the UK's main ISPs, you should be able to find the information
http://www.idl.uk.com/email/isp_settings.htm
or possibly here:
http://www.kitz.co.uk/isp/dns.htm
(If the information isn't there, you'll find it on your ISP's support pages).
Then access your PC's TCP/IP settings. The route varies depending upon your operating system.
For Windows ME (and earlier), go to Start > Settings > Dial-up Networking. Right-click on the icon for your internet connection and select 'Properties'. Click 'Networking' and 'TCP/IP Settings'.
For XP see here:
http://www.mediacollege.com/computer/network/d ns.html
For Vista see here:
http://windowshelp.microsoft.com/Windows/en-US /help/e070bf7b-6d5e-4f49-b4f7-10aa8d8b11e21033 .mspx
(You won't need most o