This is a fairly common problem, both with dial-up and broadband. The cause seems to be a registry problem which affects your PC's ability to properly connect with your ISP's DNS (= "domain name server").
The domain name server is the computer which converts user-friendly URLs, (such as www.google.com) into true 'logical addresses' (such as 74.125.45.100, which is Google's 'real' address).
A few people have found that the only way to fix the problem is to use the repair disk, in an effortto fix the registry error. But the following simpler method seems to work for most people:
First, find out the primary and secondary DNS addresses used by your ISP. (Don't panic! That might sound complicated but they're just a couple of sets of numbers, such as 194.72.6.57). If you're with one of the UK's big ISPs, the information you require will be here
http://www.idl.uk.com/email/isp_settings.htm
and/or here:
http://www.kitz.co.uk/isp/dns.htm
Then follow the instructions here but, when you get to Section 6, substitute the primary and secondary DNS addresses for your own ISP (from the above links), rather than the PlusNet ones which are shown on this page:
http://www.plus.net/support/domains_and_dns/dn s/changing_dns_vista.shtml
Chris