You have to look carefully at it before deciding whether to reuse it or not. The stock Intel pad is pretty much like a thick paste these days and if it can be respread around easily it can be reused. If in doubt, clean it off and replace it. Intel no longer offers the patch type pads in their online store, they have been replaced by a mini-syringe. If replacing the material, it's recommended to use this stuff but you *should* be able to use a premium compound such as Arctic Silver - however, that might possibly affect the processor warranty.
http://www.shop-intel.com/shop/produ...SFFMKDNX3A2TSE