If you need to renew the IP Address on a Windows Vista machine you will need to run the command prompt with administrator rights. This appears to be as the release part of this command id an admin only function.
To do this:
Press "Windows" button.
Type "CMD"
Right Click "CMD.EXE" and Select "Run as administrator"
Select "Continue" on the "User Account Control" Window
Run command as normal and it should release and renew IP address.
