[elootos]

Purevideo is a software MPEG codec that is an option to you if your system does not have an MPEG codec already (like XP Media Center).

Vista has an MPEG codec of its own so Purevideo has been phased out by nVidia.

Purevideo HD is a hardware form of it that nVidia have put on its current graphics cards that help a less powerful PC decode HD material. It is a next-generation of Purevideo that is now solely hardware based. If your PC is powerful enough then you don't need Hardware Acceleration enabled, in fact I find I have better performance without it.

This is my understanding of it all. Hope it helps.