Any user using IE8 will get a message saying your browser is not supported. Some new features may not function, but most of it probably will. XP users are encouraged to use Chrome or Firefox, which is a smart move anyway because IE8 is to easily exploitable. Windows Vista and 7 should have already upgraded to IE9, but I would still encourage the use of Chrome or Firefox over IE.
As always, call for support if needed. We will be happy to help you find a method that works for you.