Clear DNS Cache in XP and Vista to Speed Internet

Windows XP and Vista store the DNS information of Web sites you've visited, to reach those sites faster each time you access them. The cache sometimes gets corrupted or stores a lot of unusable data, and that leads to slower Internet response times. To clear the cache, go to the command prompt and type ipconfig /flushdns. The command purges the DNS cache and restores your Internet access speed."

via PC Magazine