If you are using vista and you have UAC (User Account Control) turned on, then you will need to run the command prompt with elevated permissions by right clicking "Command Prompt" and selecting "Run as Administrator", then as Spudnik says type in "ipconfig /flushdns".
UAC is on by default unless you specifically turn it off. If you don't have UAC turned on then you don't need to right click "Command Prompt".