It is sometimes necessary to reset and clear the cache in Windows 7 when you know that DNS changes have been made but Windows 7 is still resolving a hostname to the old IP address.
To refresh the DNS, you simple need to run the ipconfig /flushdns
command from the command prompt, the command prompt can be run from the Start Menu > All Programs > Accessories > Command Prompt application.
If when you run the ipconfig /flushdns
command that you receive a message saying "The requested operation requires elevation" then you need to make sure that you run the Command Prompt with administrator privledges, which is as simple as right clicking on the application when you go to run it and choosing to "Run as administrator".
For more comprehensive information check out the Flush DNS - What's My DNS?
page which contains more useful information as well as commands for other operating systems including Linux and Mac OS X.